Liquid crystal display device

ABSTRACT

A liquid crystal display device includes a first substrate; a second substrate; a liquid crystal layer interposed between the first substrate and the second substrate; a first polarizer provided on a surface of the first substrate which is on the opposite side to the liquid crystal layer; a second polarizer provided on a surface of the second substrate which is on the opposite side to the liquid crystal layer; a first phase compensation element provided between the first polarizer and the liquid crystal layer; and a second phase compensation element provided between the second polarizer and the liquid crystal layer. A plurality of pixel areas are provided for display. The first substrate includes at least one transmissive electrode, and the second substrate includes a reflective electrode region and a transmissive electrode region in correspondence with each of the plurality of pixel areas.

This is a continuation-in-part application of application Ser. No.09/122,756 filed on Jul. 27, 1998 now U.S. Pat. No. 6,195,140.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a reflective liquid crystal displaydevice and a liquid crystal display device operable both in a reflectionmode and a transmission mode, and which are used for office automationequipment such as wordprocessors and personal computers, mobileinformation devices such as hand-held computers, and VTRs integratedwith a camera and having a liquid crystal monitor. The present inventionalso relates to a method for producing such liquid crystal displaydevices. In this specification, a liquid crystal display device will bereferred to as an “LCD device”. A liquid crystal display device operableboth in a reflection mode and a transmission mode will be referred to asa “transmission- and reflection-type LCD device”.

2. Description of the Related Art

LCD devices do not emit light themselves unlike CRTs (cathode ray tubes)and EL (electroluminescence) devices. Accordingly, transmissive LCDdevices equipped with a backlight on a rear surface thereof are used.

The backlight usually consumes 50% or more of the total powerconsumption of the LCD device. Some mobile information devices which areoften used outdoors or constantly carried by the user include areflective LCD device which includes a reflective plate and performsdisplay using only the ambient light.

Reflective LCD devices include TN (twisted nematic) mode devices and STN(super twisted nematic) mode devices which use a polarizer and are in awide use as transmissive LCD devices today, as well as phase change (PC)guest-host mode devices which have been actively developed recently. ThePC guest-host mode devices do not use a polarizer and thus realizebrighter display. Such a device is disclosed in, for example, JapaneseLaid-Open Publication No. 4-75022 corresponding to U.S. Pat. No.5,220,444 and Japanese Laid-Open Publication No. 9-133930.

However, the PC guest-host mode LCD devices perform display usingoptical absorption by dyes in a liquid crystal layer including liquidcrystal molecules and the dyes dispersed therein. Accordingly, the phasetransition guest-host mode LCD devices provide significantly lowerquality than the TN devices and the STN devices using a polarizer.

In LCD devices including the liquid crystal molecules aligned inparallel or in a twisted manner, the liquid crystal molecules at thecenter and in the vicinity of the liquid crystal layer tilt verticallyto surfaces of substrates. However, the liquid crystal molecules in thevicinity of alignment layers do not tilt vertically to the surfaces ofthe substrates. Accordingly, the birefringence of the liquid crystallayer cannot be 0. Therefore, in the case where the LCD device operatesin a display mode for performing black display when a voltage isapplied, satisfactory black display is not performed due to theremaining birefringence. Thus, sufficient contrast ratio is notobtained.

The TN mode and STN mode devices do not provide sufficiently highquality display in terms of brightness and contrast. Accordingly,further improvement in the brightness and the contrast is demanded.

Reflective LCD devices are disadvantageous in that the intensity of thereflected light used for display is lowered when the ambient light isdark. By contrast, transmissive LCD devices are disadvantageous in thatthe visibility is lowered when the ambient light is very bright, forexample, outdoors on a fine day.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a liquid crystal displaydevice includes a first substrate; a second substrate; a liquid crystallayer interposed between the first substrate and the second substrate; afirst polarizer provided on a surface of the first substrate which is onthe opposite side to the liquid crystal layer; a second polarizerprovided on a surface of the second substrate which is on the oppositeside to the liquid crystal layer; a first phase compensation elementprovided between the first polarizer and the liquid crystal layer; and asecond phase compensation element provided between the second polarizerand the liquid crystal layer. A plurality of pixel areas are providedfor display. The first substrate includes at least one transmissiveelectrode, and the second substrate includes a reflective electroderegion and a transmissive electrode region in correspondence with eachof the plurality of pixel areas.

In one embodiment of the invention, each of the plurality of pixel areashas a reflective region for performing display using reflected light anda transmissive region for performing display using transmitted light,and the reflective electrode region defines the reflective region andthe transmissive electrode region defines the transmissive region.

In one embodiment of the invention, the liquid crystal layer has aretardation of zero when a molecular axis of liquid crystal molecules inthe liquid crystal layer is substantially vertical with respect to thesurface of the first and second substrates, and the first phasecompensation element and the second phase compensation element each havea retardation which fulfills λ/4 condition.

In one embodiment of the invention, the liquid crystal layer has aretardation of a when a molecular axis of liquid crystal molecules inthe liquid crystal layer is almost vertical with respect to the surfaceof the first and second substrates, and the first phase compensationelement has a retardation which fulfills λ/4-α condition.

In one embodiment of the invention, the liquid crystal layer has aretardation of a when a molecular axis of liquid crystal molecules inthe liquid crystal layer is almost vertical with respect to the surfaceof the first and second substrates, the first phase compensation elementhas a retardation which fulfills λ/4-α condition, and the second phasecompensation element has a retardation which fulfills λ/4-(β-α)condition.

In one embodiment of the invention, the first phase compensation elementand the second phase compensation element are each formed of a λ/4 waveplate, a transmission axis of the first polarizer and the first phasecompensation element make an angle of about 45 degrees, and atransmission axis of the second polarizer and the second phasecompensation element make an angle of about 45 degrees.

In one embodiment of the invention, the second phase compensationelement is formed of a λ/4 wave plate, and a slower optic axis of thesecond phase compensation element matches one of a longer axis or ashorter axis of elliptically polarized light transmitted through theliquid crystal layer and incident on the second phase compensationelement so as to convert the elliptically polarized light into linearlypolarized light, and a transmission axis of the second polarizer isperpendicular to a polarizing axis of the linearly polarized light.

According to another aspect of the invention, a liquid crystal displaydevice includes a first substrate including a transmissive electrode; asecond substrate including a reflective electrode; a liquid crystallayer interposed between the first substrate and the second substrateand including liquid crystal molecules which exhibit negative dielectricanisotropy and are aligned substantially vertically to surfaces of thefirst substrate and the second substrate when no voltage is applied; apolarizer provided on a surface of the first substrate which is oppositeto the liquid crystal layer; and a λ/4 wave plate provided between thepolarizer and the liquid crystal layer. A slower axis of the λ/4 waveplate and a transmission axis of the polarizer make an angle of about 45degrees.

In one embodiment of the invention, the liquid crystal display devicefurther includes a phase compensation element between the reflectionelectrode and the polarizer.

According to still another aspect of the invention, a liquid crystaldisplay device includes a first substrate; a second substrate; a liquidcrystal layer interposed between the first substrate and the secondsubstrate and including liquid crystal molecules which exhibit negativedielectric anisotropy and are aligned substantially vertically tosurfaces of the first substrate and the second substrate when no voltageis applied; a first polarizer provided on a surface of the firstsubstrate which is on the opposite side to the liquid crystal layer; asecond polarizer provided on a surface of the second substrate which ison the opposite side to the liquid crystal layer; a first λ/4 wave plateprovided between the first polarizer and the liquid crystal layer; and asecond λ/4 wave plate provided between the second polarizer and theliquid crystal layer. A plurality of pixel areas are provided fordisplay. The first substrate includes at least one transmissiveelectrode, and the second substrate includes a reflective electroderegion and a transmissive electrode region in correspondence with eachof the plurality of pixel areas. Slower axes of the first λ/4 wave plateand the second λ/4 wave plate are in an identical direction and make anangle of about 45 degrees with each of transmission axes of the firstpolarizer and the second polarizer.

In one embodiment of the invention, each of the plurality of pixel areashas a reflective region for performing display using reflected light anda transmissive region for performing display using transmitted light,and the reflective electrode region defines the reflective region andthe transmissive electrode region defines the transmissive region.

In one embodiment of the invention, the liquid, crystal display devicefurther includes at least one phase compensation element between thefirst polarizer and the second polarizer.

In one embodiment of the invention, the liquid crystal layer furtherincludes a chiral dopant.

In one embodiment of the invention, the liquid crystal layer has anapproximately 90 degree twisted orientation.

In one embodiment of the invention, the first polarizer and the secondpolarizer have transmission axes perpendicular to each other, and thefirst phase compensation element and the second phase compensationelement have slower axes perpendicular to each other.

In one embodiment of the invention, the first phase compensation elementconverts linearly polarized light from the first polarizer intocircularly polarized light, and the second phase compensation elementconverts linearly polarized light from the second polarizer intocircularly polarized light, the liquid crystal display device furtherincluding a third phase compensation element provided between the firstpolarizer and the liquid crystal layer for compensating for wavelengthdependency of refractive index anisotropy of the first phasecompensation element.

In one embodiment of the invention, the third phase compensation elementis a λ/2 wave plate, and when a transmission axis of the first polarizerand a slower axis of the third phase compensation element make an angleof γ1, the transmission axis of the first polarizer and a slower axis ofthe first phase compensation element make an angle of 2γ1+45 degrees.

In one embodiment of the invention, the liquid crystal display devicefurther includes a fourth phase compensation element provided betweenthe second polarizer and the liquid crystal layer for compensating forwavelength dependency of refractive index anisotropy of the second phasecompensation element.

In one embodiment of the invention, the fourth phase compensationelement is a λ/2 wave plate, and when a transmission axis of the secondpolarizer and a slower axis of the fourth phase compensation elementmake an angle of γ2, the transmission axis of the second polarizer and aslower axis of the second phase compensation element make an angle of2γ2+45 degrees.

In one embodiment of the invention, the transmission axis of the firstpolarizer is perpendicular to the transmission axis of the secondpolarizer, a slower axis of the first phase compensation element isperpendicular to the slower axis of the second phase compensationelement, and a slower axis of the third phase compensation element isperpendicular to the slower axis of the fourth phase compensationelement.

According to still another aspect of the invention, a liquid crystaldisplay device includes a first substrate; a second substrate; and aliquid crystal layer interposed between the first substrate and thesecond substrate. A plurality of pixel areas are provided for display,each of the plurality of pixel areas having a reflective region forperforming display using reflected light and a transmissive region forperforming display using transmitted light. The first substrate includesa counter electrode in the vicinity of the liquid crystal layer. Thesecond substrate includes, in the vicinity of the liquid crystal layer,a plurality of gate lines, a plurality of source lines perpendicular tothe plurality of gate lines, a plurality of switching elements providedin the vicinity of intersections of the plurality of gate lines and theplurality of source lines, a first conductive layer having a high lighttransmission efficiency, and a second conductive layer having a highlight reflection efficiency, the first conductive layer and the secondconductive layer being connected to each of the switching elements,connected to each other, and being provided in each of the pixel areas.

In one embodiment of the invention, the liquid crystal display devicefurther includes an insulating layer between the first conductive layerand the second conductive layer.

In one embodiment of the invention, the second substrate furtherincludes a third conductive layer, and the first conductive layer andthe second conductive layer are connected to each other through thethird conductive layer.

In one embodiment of the invention, one of the first conductive layer,the second conductive layer and the third conductive layer is formed ofa material identical with one of materials forming the plurality of gateelectrodes or the plurality of source electrodes.

In one embodiment of the invention, the insulating layer has a wave-likesurface below the second conductive layer.

According to still another aspect of the invention, a method forproducing a liquid crystal display device is provided. The liquidcrystal display device includes a first substrate; a second substrate;and a liquid crystal layer interposed between the first substrate andthe second substrate. A plurality of pixel areas are provided fordisplay, each of the plurality of pixel areas having a reflective regionfor performing display using reflected light and a transmissive regionfor performing display using transmitted light. The first substrateincludes a counter electrode in the vicinity of the liquid crystallayer. The second substrate includes, in the vicinity of the liquidcrystal layer, a plurality of gate lines, a plurality of source linesperpendicular to the plurality of gate lines, a plurality of switchingelements provided in the vicinity of intersections of the plurality ofgate lines and the plurality of source lines, a first conductive layerhaving a high light transmission efficiency, a second conductive layerhaving a high light reflection efficiency, the first conductive layerand the second conductive layer being connected to each of the switchingelements, connected to each other, and being provided in each of thepixel areas, and an insulating layer provided between the firstconductive layer and the second conductive layer. The method includesthe steps of forming the first conductive layer on a plate; forming theinsulating layer at least on the first conductive layer; forming thesecond conductive layer on the insulating layer; and partially removingthe second conductive layer formed on the first conductive layer.

In one embodiment of the invention, the method further includes thesteps of forming a third conductive layer on a connection area, on atleast the first conductive layer, for connecting the first conductivelayer and the second conductive layer so as to connect the firstconductive layer and the second conductive layer to each other throughthe third conductive layer; forming the insulating layer; and partiallyremoving the insulating layer at least on the connection area forconnecting the first conductive layer and the second conductive layer.

In one embodiment of the invention, the step of partially removing theinsulating layer includes the step of removing the insulating layer onan area of the first conductive layer.

Thus, the invention described herein makes possible the advantages ofproviding a reflection-type LCD device and a transmission- andreflection-type LCD device providing satisfactory display with asufficiently high contrast, and a method for producing the same.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a reflection-type LCD device in an exampleaccording to the present invention;

FIG. 2 is a schematic view of a transmission- and reflection-type LCDdevice in an example according to the present invention;

FIG. 3 is a schematic view of a transmission- and reflection-type LCDdevice in an example according to the present invention;

FIG. 4 is a graph illustrating a spectral reflectance characteristics ofa reflection-type LCD device according to the present invention having agap of d=3.56 μm obtained when the light is incident and receivedvertically;

FIG. 5 is a graph illustrating a spectral reflectance characteristics ofa reflection-type LCD device according to the present invention having agap of d=4.5 μm obtained when the light is incident and receivedvertically;

FIG. 6 is a graph illustrating the relationship between the cell gap andthe contrast ratio of a reflection-type LCD device according to thepresent invention obtained when the light is incident and receivedvertically at a wavelength of 550 nm;

FIG. 7A is a plan view of an active matrix substrate in a first exampleaccording to the present invention;

FIG. 7B is a cross-sectional view of the active matrix substrate takenalong line 7B-7B′ of FIG. 7A;

FIG. 8A is a plan view of an active matrix substrate in a second exampleaccording to the present invention;

FIG. 8B is a cross-sectional view of the active matrix substrate takenalong line 8B-8B′ of FIG. 8A;

FIG. 8C is a plan view of an active matrix substrate used in asemi-transmission- and semi-reflection-type LCD device according to thepresent invention;

FIG. 8D is a cross-sectional view of the active matrix substrate takenalong line 8D-8D′ of FIG. 8C;

FIG. 8E is a cross-sectional view of the active matrix substrate takenalong line 8E-8E′ of FIG. 8C;

FIG. 9 is a graph illustrating a spectral reflectance characteristics ofa transmission- and reflection-type LCD device according to the presentinvention having a gap of d=3.56 μm obtained when the light is incidentand received vertically;

FIG. 10 is a graph illustrating a spectral reflectance characteristicsof a transmission- and reflection-type LCD device according to thepresent invention having a gap of d=4.5 μm obtained when the light isincident and received vertically;

FIG. 11 is a graph illustrating the relationship between the cell gapand the contrast ratio of a transmission- and reflection-type LCD deviceaccording to the present invention obtained when the light is incidentand received vertically at a wavelength of 550 nm;

FIG. 12 is a graph illustrating the relationship between the angle ofthe slower optic axis of the λ/4 wave plate and the contrast ratio inthe first example;

FIG. 13A is a schematic view illustrating light transmission andreflection in black display in the LCD device according to the presentinvention;

FIG. 13B is a schematic view illustrating light transmission andreflection in white display in the LCD device in the second exampleaccording to the present invention;

FIG. 14A is a schematic view illustrating light transmission andreflection in black display in the reflection mode in an LCD device in afourth example according to the present invention;

FIG. 14B is a schematic view illustrating light transmission andreflection in white display in the reflection mode in the LCD device inthe fourth example according to the present invention;

FIG. 15A is a schematic view illustrating light transmission andreflection in black display in the transmission mode in the LCD devicein the fourth example according to the present invention;

FIG. 15B is a schematic view illustrating light transmission andreflection in white display in transmission mode in the LCD device inthe fourth example according to the present invention;

FIG. 16 is a graph illustrating the relationship between the wavelengthand the transmittance in black display in the fourth example;

FIG. 17 is a schematic view of a reflection-type LCD device in a fifthexample according to the present invention;

FIG. 18A is a schematic view illustrating light transmission andreflection in black display in the reflection mode in the LCD device inthe fifth example according to the present invention;

FIG. 18B is a schematic view illustrating light transmission andreflection in white display in the reflection mode in the LCD device inthe fifth example according to the present invention;

FIG. 18C is a schematic view illustrating light transmission andreflection in black display in the transmission mode in the LCD devicein the fifth example according to the present invention;

FIG. 18D is a schematic view illustrating light transmission andreflection in white display in transmission mode in the LCD device inthe fifth example according to the present invention;

FIG. 19 is a graph illustrating the relationship between the wavelengthand the transmittance in black display in the fifth example;

FIG. 20 is a graph illustrating the relationship between the wavelengthand the transmittance in black display in the fifth example;

FIG. 21 is a plan view of an active matrix substrate of an LCD device ina sixth example according to the present invention;

FIG. 22 is a cross-sectional view of the active matrix substrate shownin FIG. 21;

FIGS. 23A through 23E are cross-sectional views illustrating a methodfor forming the active matrix substrate shown in FIGS. 21 and 22;

FIG. 24 is a plan view of an active matrix substrate of an LCD device ina seventh example according to the present invention;

FIG. 25 is a cross-sectional view of the LCD device taken along line25-25′ in FIG. 24;

FIGS. 26A through 26C are cross-sectional views illustrating a methodfor forming an active matrix substrate of an LCD device in an eighthexample according to the present invention;

FIGS. 27A through 27C are cross-sectional views illustrating a methodfor forming an active matrix substrate of an LCD device in a ninthexample according to the present invention;

FIGS. 28A through 28C are cross-sectional views illustrating a methodfor forming an active matrix substrate of an LCD device in a tenthexample according to the present invention;

FIGS. 29A through 29C are cross-sectional views illustrating a methodfor forming an active matrix substrate of an LCD device in an eleventhexample according to the present invention;

FIG. 30 is a plan view of the active matrix substrate of the LCD devicein the eleventh example;

FIGS. 31A through 31E and 32A through 32C are cross-sectional viewsillustrating a method for forming a display section of the LCD device inthe eleventh example;

FIGS. 33A through 33F are cross-sectional views illustrating a methodfor forming a gate terminal section of the LCD device in the eleventhexample;

FIG. 34A is a cross-sectional view of a gate terminal section of an LCDdevice in a modification of the eleventh example;

FIG. 34B is a cross-sectional view of a source terminal section of anLCD device in a modification of the eleventh example;

FIGS. 35A through 35C are cross-sectional view of a method for producingan LCD device in a twelfth example according to the present invention;

FIG. 36 is a cross-sectional view of an LCD device produced by onemethod in a thirteenth example according to the present invention;

FIG. 37A is a cross-sectional view of an LCD device produced by anothermethod in a thirteenth example according to the present invention;

FIG. 37B is a graph illustrating the voltage-brightness relationship ofthe LCD device shown in FIG. 37A;

FIG. 38A is a cross-sectional view of an LCD device produced by acomparative method; and

FIG. 38B is a graph illustrating the voltage-brightness relationship ofthe LCD device shown in FIG. 38A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The terms “reflective electrode region”, “transmissive electroderegion”, “reflective region” and “transmissive region” will bedescribed.

A reflection-type LCD device for performing display using ambient lighthas a reflective electrode region on one of two substrates forreflecting the ambient light transmitted through a liquid crystal layer.The reflective electrode region can be formed of a reflective electrodeor a combination of a transmissive electrode and a reflective layer(e.g., reflective plate). In other words, an electrode for applying avoltage to the liquid crystal layer can be a transmissive electrode, andin this case, the reflective electrode region for reflecting input lightneed not act as an electrode.

A transmission- and reflection-type LCD device has a reflectiveelectrode region, like the reflection-type LCD device. The reflectiveelectrode region can be formed a reflective electrode or a combinationof a transmissive electrode and a reflective layer (e.g., reflectiveplate). The transmissive electrode region is typically formed of atransmissive electrode. In the case of an LCD device having relativelysmall transmissive electrode regions in the reflective electrode(referred to as a “semi-transmission- and semi-reflection-type LCDdevice), the liquid crystal molecules are driven by a voltage applied tothe liquid crystal layer by the reflective electrode in the transmissiveelectrode region. Therefore, the transmissive electrode regions need notact as electrodes. In this type of LCD device, each of the transmissiveelectrode regions has one dimension smaller than the thickness of theliquid crystal layer (e.g., diameter in the case where the transmittanceelectrode region is a circle). In the case where the reflectiveelectrode region is formed of a combination of a transmissive electrodeand a reflective layer is used in the semi-transmission- andsemi-reflection-type LCD device), a transmissive electrode formed in theentirety of each pixel area and a reflective layer having a plurality ofopenings can be used.

In the transmission- and reflection-type LCD device (excluding thesemi-transmission- and semi-reflection-type LCD device) according to thepresent invention, the region for performing display in a transmissionmode is referred to as a “transmissive region”, and the region forperforming display in a reflection mode is referred to as a “reflectiveregion”. The transmissive region and the reflective region each includesa transmissive electrode region, a reflective electrode region and aliquid crystal layer defined by the transmissive electrode region andthe reflective electrode region.” The semi-transmission- andsemi-reflection-type LCD device also includes a reflective electroderegion and a transmissive electrode region, but light reflected by thereflective electrode region and light transmitted through thetransmissive electrode region are mixed and overlapped. Accordingly, thetransmissive region and the reflective region are not independentlydefined. In other words, among the LCD devices having a transmissiveelectrode region and a reflective electrode region for each pixel area,an LCD device in which the transmissive region and the reflective regioncannot be independently defined (i.e., the regions are substantiallyoverlapped) is referred to as the semi-transmission- andsemi-reflection-type LCD device.

The term “pixel area” is defined as follows. An LCD device according tothe present invention has a plurality of pixel areas for performingdisplay. The term “pixel area” is defined as a part (an element) of anLCD device forming a pixel which is a minimum unit of display.Typically, in an active matrix LCD device including a counter electrodeand a plurality of pixel electrodes arranged in a matrix and switched byeach of active elements (e.g., TFTs), a pixel area includes one of thepixel electrodes, an area of the counter electrode positionallycorresponding to the pixel electrode, and an area of the liquid crystallayer interposed therebetween. In a simple matrix LCD device includingstriped electrodes (scanning electrodes and signal electrodes)respectively formed in two substrates and arranged to cross each otherwith the liquid crystal layer being interposed therebetween, a pixelarea includes a crossing area at which the striped electrodes cross eachother and an area of the liquid crystal layer positionally correspondingto the crossing area. A transmission- and reflection-type LCD deviceaccording to the present invention has a reflective electrode region anda transmissive electrode region for each pixel area.

The “phase compensation element” includes a phase plate or a phase film,and the “polarizer” includes a polarizing plate or a polarizing film.

The “retardation” refers to a retardation with respect to the lightincident vertically on the liquid crystal layer or a phase compensationelement unless otherwise specified.

Embodiment 1

In a first embodiment of the present invention, a reflection-type LCDdevice having a higher display quality than the conventional LCD deviceis provided as specifically described in a first example. Areflection-type LCD device in the first embodiment, as shown in FIG. 1,includes a first substrate 1 including a transmissive electrode 4, asecond substrate 2 including a reflective electrode region 3, a liquidcrystal layer 5 interposed between the first substrate 1 and the secondsubstrate 2, a polarizer 6 provided on a surface of the first substrate1 opposite to the liquid crystal layer 5, and a λ/4 wave plate 7provided between the polarizer 6 and the liquid crystal layer 5. FIG. 1schematically shows one pixel area of the reflection-type LCD deviceaccording to the present invention.

The liquid crystal layer 5 includes liquid crystal molecules (not shown)exhibiting negative dielectric anisotropy. The liquid crystal moleculesin the liquid crystal layer 5 are substantially treated so as to alignvertically to surfaces of the first and second substrates 1 and 2 whenno voltage is applied. (A liquid crystal layer treated so that theliquid crystal molecules therein are aligned vertically to the surfacesof the substrate when no voltage is applied will be referred to as a“vertically aligned liquid crystal layer”. A liquid crystal layertreated so that the liquid crystal molecules therein are alignedhorizontal to the surfaces of the substrate when no voltage is appliedwill be referred to as a “horizontally aligned liquid crystal layer”.) Aslower optic axis of the λ/4 wave plate 7 and a transmission axis of thepolarizer 6 are set to make an angle of about 45 degrees. The linearlypolarized light incident through the polarizer 6 is converted intocircularly polarized light, and the circularly polarized light reflectedby the reflective electrode region 3 and transmitted through the liquidcrystal layer 5 is converted into the linearly polarized light.Accordingly, the retardation by the liquid crystal layer 5 issubstantially zero when no voltage is applied, and thus satisfactoryblack display is obtained.

A transmission- and reflection-type LCD device in the first embodiment,as shown in FIG. 2, includes a first substrate 1, a second substrate 2,a liquid crystal layer 5 interposed between the first and secondsubstrates 1 and 2, a first polarizer 6 provided on a surface of thefirst substrate 1 opposite to the liquid crystal layer 5, and a secondpolarizer 9 provided on a surface of the second substrate 2 opposite tothe liquid crystal layer 5. The transmission- and reflection-type LCDdevice further includes a first phase compensation element 7 (typically,a λ/4 wave plate) provided between the first polarizer 6 and the liquidcrystal layer 5 and a second phase compensation element 10 (typically, aλ/4 wave plate) provided between the second polarizer 9 and the liquidcrystal layer 5. The second substrate 2 includes a reflective electroderegion (R) 3 and a transmissive electrode region 8 (T) for each of aplurality of pixel areas. FIG. 2 schematically shows one pixel area ofthe transmission- and reflection-type LCD device. In FIG. 2, thereflective electrode region 3 (R) and the transmissive electrode region8 (T) are each shown as one region for simplicity. The transmission- andreflection-type LCD device according to the present invention is notlimited to this and can have a plurality of transmissive electroderegion 8 in the reflective electrode region 3 like a semi-transmission-and semi-reflection-type LCD device.

The transmission- and reflection-type LCD device shown in FIG. 2operates in the following manner.

In the reflection mode, when the retardation (birefringence) of theliquid crystal layer in the viewing direction (thickness direction ofthe liquid crystal layer) is substantially zero (that is, initialalignment state in the vertical alignment mode and the state of beingsupplied with a prescribed saturation voltage in the horizontalalignment state), black (dark) display is performed by the followingreason. The linearly polarized light transmitted through the firstpolarizer is transmitted through the first phase compensation elementand the liquid crystal layer, then reflected, and again transmittedthrough the liquid crystal layer and the first phase compensationelement to be incident on the first polarizer. At this point, the lighthas sufficient polarizing components perpendicular to the transmissionaxis of the first polarizer to perform black display.

When there is a retardation in the viewing direction, the white (bright)display is performed for the following reason. The linearly polarizedlight transmitted through the first polarizer is transmitted through thefirst phase compensation element and the liquid crystal layer, thenreflected, and again transmitted through the liquid crystal layer andthe first phase compensation element to be incident on the firstpolarizer. At this point, the light has sufficient polarizing componentsparallel to the transmission axis of the first polarizer to performwhite display. Gray scale display corresponding to various retardationsby the liquid crystal layer is realized by an application of voltageacross the liquid crystal layer.

In the transmission mode, when the retardation in the viewing directionis substantially zero, black display is performed by the followingreason. The linearly polarized light transmitted through the secondpolarizer is transmitted through the second phase compensation element,the liquid crystal layer and the first phase compensation element to beincident on the first polarizer. At this point, the light has sufficientpolarizing components perpendicular to the transmission axis of thefirst polarizer to perform black display.

When there is a retardation in the viewing direction, the white displayis performed for the following reason. The linearly polarized lighttransmitted through the second polarizer is transmitted through thesecond phase compensation element, the liquid crystal layer, and thefirst phase compensation element to be incident on the first polarizer.At this point, the light has sufficient polarizing components parallelto the transmission axis of the first polarizer to perform whitedisplay. Gray scale display corresponding to various retardations isrealized.

Accordingly, when the reflection mode and the transmission mode are usedtogether, black display is performed in both modes, realizing highcontrast display. The gray scale display is performed by changing theretardation by controlling the voltage.

In the case where the reflective region for performing display in thereflection mode and the transmissive region for performing display inthe transmission mode are formed for each pixel area, the utilizationfactor of the reflected light is improved. Furthermore, in such astructure, the thicknesses of the liquid crystal layer (retardation) inthe reflective region and the transmissive region are independentlyadjusted. Thus, each of the display modes is optimized.

The retardation by the liquid crystal layer is substantially zero whenthe molecular axis of the liquid crystal molecules is substantiallyvertical to the surfaces of the first and second substrates. When theretardation by each of the first phase compensation element and thesecond phase compensation element fulfills the λ/4 condition, there isalmost no birefringence by the liquid crystal layer in the viewingdirection in the reflection mode using the light reflected by the arearegion having a reflection function such as a reflective layer orreflective plate. Accordingly, the circularly polarized light isincident on and reflected by the reflective electrode region to becircularly polarized light having an opposite rotation direction. Thelight is transmitted through the first phase compensation element to belinearly polarized light perpendicular to the transmission axis of thefirst polarizer. Since the reflective region acts as a light isolator,black display with very little optical leakage is provided.

When there is a retardation (birefringence) by the liquid crystal layerin the viewing direction in the reflection mode, the retardation can bechanged by controlling the voltage, so that the light which has beenincident on the first polarizer, reflected and again incident on thefirst polarizer has components parallel to the transmission axis of thefirst polarizer. Accordingly, bright display having gray scale isprovided.

When there is almost no retardation by the liquid crystal layer in theviewing direction in the transmission mode, the circularly polarizedlight incident on the liquid crystal layer is maintained as beingcircularly polarized when being transmitted through the liquid crystallayer. The light is transmitted through the first phase compensationelement to be linearly polarized light perpendicular to the transmissionaxis of the first polarizer. Thus, black display with very littleoptical leakage is provided.

When there is a retardation by the liquid crystal layer in the viewingdirection in the transmission mode, the retardation can be changed bycontrolling the voltage. Thus, the light incident on the secondpolarizer is incident on the first polarizer as being parallel to thetransmission axis of the first polarizer. Thus, the white display havinggray scale is provided.

As described above, when both the reflection mode and the transmissionmode are used, the state of the liquid crystal molecules for blackdisplay is the same in both display modes, and the black display withsubstantially no optical leakage is provided. Regardless of the ambientlight intensity, the transmission- and reflection-type LCD deviceprovides high contrast display.

In such an LCD device, even when the retardation (α) caused by theliquid crystal molecules remaining in the state of being almost verticalto the first and second substrates is not negligible, for example, whena horizontally aligned liquid crystal layer is used or when the pretiltangle is excessively large in a vertically aligned liquid crystal layer,high contrast display is provided in the reflection mode by setting theremaining retardation by the liquid crystal layer and the retardation bythe phase compensation elements in combination to fulfill the λ/4condition in a wide wavelength range.

In the reflection mode, the light going out of the liquid crystal layerafter being transmitted through the liquid crystal layer twice iselliptically polarized light which is offset from the circularlypolarized light by the remaining retardation α. The ellipticallypolarized light is phase-offset by 90 degrees from the light which wasincident. Accordingly, when transmitted through the first phasecompensation element having a retardation of λ/4-α, the light becomeslinearly polarized light perpendicular to the transmission axis of thefirst polarizer. Since the reflective region acts as a light isolator,black display with very little optical leakage is provided.

As can be appreciated, even when the remaining retardation is notnegligible, high contrast display is obtained in the reflection mode. Inthe case where mainly reflection-mode display is performed, such as, forexample, when the reflective pixel electrodes are larger than thetransmissive pixel electrodes, the second phase compensation element 10shown in FIG. 3 can be a λ/4 wave plate.

Even in the case where the remaining retardation is not negligible, forexample, when a horizontally aligned liquid crystal layer is used orwhen the pretilt angle is excessively large in a vertically alignedliquid crystal layer, high contrast display is provided for areflection-type LCD device, a transmission-type LCD device and atransmission- and reflection-type LCD device by the following structure.Where α is the retardation by the liquid crystal layer in the reflectiveregion when the liquid crystal molecules are aligned almost verticallyto the substrates and β is the retardation by the liquid crystal layerin the transmissive region, the retardation of the first phasecompensation element fulfills λ/4-α and the retardation of the secondphase compensation element fulfills λ/4-(β-α).

As described above, in the reflection mode, the light going out of theliquid crystal layer after being transmitted through the liquid crystallayer twice is elliptically polarized light which is offset from thecircularly polarized light by the remaining retardation α. The lightbecomes linearly polarized light perpendicular to the transmission axisof the first polarizer when transmitted through the first phasecompensation element having a retardation of λ/4-α.

In the transmission mode, when the retardation (β) caused by the liquidcrystal molecules in the viewing angle is not negligible, the lightgoing out of the liquid crystal layer is elliptically polarized light asin the reflection mode because the second phase compensation element isset to have a retardation of λ/4-(β-α). The elliptically polarized lightbecomes linearly polarized light perpendicular to the transmission axisof the first polarizer when transmitted through the phase compensationelement. Thus, black display with very little optical leakage isobtained.

Even in the case where the remaining retardation is not negligible, highcontrast display is provided for a reflection-type LCD device, atransmission-type LCD device and a transmission- and reflection-type LCDdevice by the following structure.

When the remaining retardation is negligible, circularly polarized lightis incident on the liquid crystal layer to obtain high contrast displaywith a simplest structure due to the following setting. The first andsecond compensation elements are each formed of a λ/4 wave plate. Thetransmission axis of the first polarizer and the slower optic axis ofthe first phase compensation element are set to make an angle of about45 degrees. The transmission axis of the second polarizer and the sloweroptic axis of the second phase compensation element are set to make anangle of about 45 degrees.

In a transmission- and reflection-type LCD device, a liquid crystallayer in which the liquid crystal molecules are aligned substantiallyvertically to the surfaces of the substrates when no voltage is appliedcan be used. In such a case, the LCD device can act, when the ambientlight is dark, as a transmission-type LCD device for performing displayusing the light which is from the backlight and transmitted through thetransmissive electrode region formed of a material having a relativelyhigh light transmittance. When the ambient light is bright, the LCDdevice can be used as a reflection-type LCD device for performingdisplay using the ambient light reflected by the reflective electroderegion formed of a material having a relatively high light reflectance.When the transmission mode and the reflection mode are used together,substantially complete black display is performed in both modes. Thus,high contrast display is realized. This will be described below.

A transmission- and reflection-type LCD device is generally operable inboth the normally black (hereinafter, referred to as “NB”) mode and thenormally white (hereinafter, referred to as “NW”) mode usingbirefringence.

In the NW mode, the voltage to be applied to achieve black displaychanges as the cell gap changes. In the NB mode, the voltage to beapplied to achieve white display changes as the cell gap changes.Accordingly, in the NW mode, the contrast ratio significantly changes inaccordance with the cell gap, which requires precise cell gap control.In the NB mode, the contrast does not substantially change in accordancewith the cell gap, which provides a larger margin for cell gap control.Moreover in the NB mode, when the switching element (e.g., TFT)malfunctions to prevent voltage application on the pixel electrode, thisresults in an inconspicuous black point.

A transmission- and reflection-type LCD device operable in the NB modehas a high production efficiency, and high contrast display is realizedeasily regardless of the ambient light intensity according to thepresent invention.

A phase compensation element can be provided for compensating for theinfluence by the refractive index anisotropy of the liquid crystalmolecules caused in the light incident direction on the liquid crystallayer and the viewing direction. In such a structure, the reduction incontrast in accordance with the light incident direction and the viewingdirection is prevented.

When a chiral dopant is added to the vertically aligned liquid crystallayer formed of a liquid crystal material exhibiting negative dielectricanisotropy of the LCD device, the liquid crystal molecules are rotatedwhen a voltage is applied. Thus, the rotation of the liquid crystalmolecules at the time of voltage application is stabilized by the chiraldopant.

When the alignment layers in the vicinity of the two substrates arerubbed in different directions, the traces of the alignment treatmentare not in the same directions and thus are less conspicuous. When theliquid crystal layer has a 90 degree twist orientation, black displaywith very little optical leakage is obtained for the following reason.The tilt directions of the liquid crystal molecules in the vicinity ofthe two substrates make an angle of 90 degrees, and thus theretardations generated in the tilt directions are counteracted by eachother.

The refractive indices of birefringent materials forming the phasecompensation elements with respect to the ordinary ray and extraordinaryray strongly depend on the wavelength. Therefore, the phase delayaccumulated in the phase compensation elements at a specific thicknessalso depends on the wavelength. In other words, the phase delay (e.g.,λ/4) can be completely provided to the linearly polarized light ofincidence only when the incident light has a certain single wavelength.Accordingly, in the area where the phase delay of λ/4 is not achieveddue to the wavelength dependency of the refractive index anisotropy ofthe birefringent material forming the λ/4 wave plates, a part of thelight is transmitted through the polarizer on the outgoing side withoutbeing absorbed by polarizer. As a result, the darkness of the blackdisplay changes. According to the present invention, the slower opticaxes of the first and second phase compensation elements wave plates areset to be perpendicular to each other. Due to such a structure, thewavelength dependency of the refractive index anisotropy of the firstphase compensation element is counteracted by the wavelength dependencyof the refractive index anisotropy of the second phase compensationelement. Thus, a certain phase difference is fulfilled in the entiretyof the certain wavelength range. Thus, the darkness of the black displayis improved.

As shown in FIG. 17, a third phase compensation element 11 can beprovided between the first polarizer 6 and the liquid crystal layer 5.Due to such a structure, the wavelength dependency of the refractiveindex anisotropy caused when the linearly polarized light is convertedinto circularly polarized light by the first phase compensation elementis counteracted to some extent. Accordingly, in the reflection mode,conversion into circularly polarized light is performed in the statewhere the dispersion in the polarization state is reduced over a widewavelength range. Thus, the darkness of the black display is improved.

In the case where the third compensation element is formed of a λ/2 waveplate and the transmission axis of the first polarizer and the sloweroptic axis of the first phase compensation element are set to make anangle of 2γ1+45 degrees (where γ1 is the angle made by the transmissionaxis of the first polarizer and the slower optic axis of the thirdcompensation element), the polarization direction of the linearlypolarized light from the first compensation element is changed inorientation by the third phase compensation element and then convertedinto circularly polarized light by the first phase compensation element.Accordingly, the wavelength dependency of the refractive indexanisotropy of the first phase compensation element is compensated foroptimally. Therefore, the dispersion in the polarization state isreduced in a wide wavelength range in the reflection mode, and thuscircularly polarized light is obtained in a satisfactory manner. Thus,the darkness of the black display in the reflection mode is improved.Substantially the same effects are obtained when the first and thirdphase compensation elements are located oppositely.

Especially when a vertically aligned liquid crystal layer is used orwhen the remaining retardation by the liquid crystal layer is negligiblein the dark state, the first phase compensation element can be formed ofa λ/4 wave plate.

When a retardation of α is remaining in the liquid crystal layer in thereflection mode in the dark state, the retardation of the first phasecompensation element is made λ/4-α so that light which is offset fromthe circularly polarized light is incident on the liquid crystal layer.When the light is transmitted through the liquid crystal layer andreaches the reflective electrode, the light becomes circularly polarizedlight as a result of the dispersion in the polarization state beingeliminated over a wide wavelength range. Thus, satisfactory blackdisplay is realized in the reflection mode.

A fourth phase compensation element 12 can be provided between thesecond polarizer 9 and the liquid crystal layer 5. Due to such astructure, the wavelength dependency of the refractive index anisotropycaused when the linearly polarized light is converted into circularlypolarized light is counteracted to some extent. Accordingly, in thetransmission mode, conversion into circularly polarized light isperformed in the state where the dispersion in the polarization state isreduced in a wide wavelength range. Thus, the darkness of the blackdisplay is improved. Even when the reflection mode and the transmissionmode are used together, satisfactory black display is realized.

In the case where the fourth compensation element is formed of a λ/2wave plate and the transmission axis of the second polarizer and theslower optic axis of the second phase compensation element are set tomake an angle of 2γ2+45 degrees (where γ2 is the angle made by thetransmission axis of the second polarizer and the slower optic axis ofthe fourth compensation element), the linearly polarized light from thesecond compensation element is changed in orientation by the fourthphase compensation element and then converted into circularly polarizedlight by the second phase compensation element. Accordingly, thewavelength dependency of the refractive index anisotropy of the firstphase compensation element is compensated for optimally. Therefore, thedispersion in the polarization state is reduced in a wide wavelengthrange in the transmission mode, and thus circularly polarized light isobtained in a satisfactory manner.

Especially when a vertically aligned liquid crystal layer is used orwhen the remaining retardation by the liquid crystal layer is negligiblein the dark state, the second phase compensation element can be formedof a λ/4 wave plate.

When a retardation of α is remaining in the reflection mode and aretardation of β is remaining in transmission mode in the liquid crystallayer in the dark state, the retardations of the elements used later aremade λ/4-(β-α) so that light which is offset from the circularlypolarized light is incident on the liquid crystal layer. When the lightis transmitted through the liquid crystal layer, the light is in thesame polarization state as that in the reflection mode. Therefore, whenbeing transmitted through the third phase compensation element, thelight becomes linearly polarized light perpendicular to the transmissionaxis of the first polarizer. Thus, the darkness of the black display isimproved. Even when the transmission mode and the reflection mode areused together, satisfactory black display is realized.

In the case where the slower optic axes of the first and second phasecompensation elements to perpendicular to each other and the sloweroptic axes of the third and fourth phase compensation elements toperpendicular to each other, the wavelength dependency of the refractiveindex anisotropy of the first and third phase compensation elements iscounteracted by the wavelength dependency of the refractive indexanisotropy of the second and fourth phase compensation elements,respectively. In this manner, the darkness of the black display isimproved.

EXAMPLE 1

An LCD device in a first example according to the present invention willbe described with reference to FIG. 1.

A substrate 2 includes a reflective electrode 3 (shown as reflectiveelectrode region in FIG. 1) formed of a material having a highreflectance such as, for example, Al or Ta. A substrate 1 includes acounter electrode 4 (shown as transmissive electrode in FIG. 1). Aliquid crystal layer 5 formed of a liquid crystal material exhibitingnegative dielectric anisotropy is interposed between the reflectiveelectrode 3 and the counter electrode 4.

Alignment layers (not shown) are provided on surfaces of the reflectiveelectrode 3 and the counter electrode 4 which are in contact with theliquid crystal layer 5. The alignment layers are used to align liquidcrystal molecules (not shown) in the liquid crystal layer 5 to bevertical to the surfaces of the substrates 1 and 2. After the alignmentlayers are provided, at least one of the alignment layers is processedwith alignment treatment such as, for example, rubbing.

Due to the alignment treatment, the liquid crystal molecules in theliquid crystal layer 5 has a tilt angle of about 0.1 to 5 degrees withrespect to the vertical direction to the surfaces of the substrates 1and 2.

Since the liquid crystal layer 5 is formed of a material exhibitingnegative dielectric anisotropy, when a voltage is applied between thereflective electrode 3 and the counter electrode 4, the liquid crystalmolecules tilt to be horizontal with respect to the surfaces of thesubstrates 1 and 2.

The reflective electrode 3 is used for applying a voltage to the liquidcrystal layer 5, but the reflective electrode 3 can be used only as areflective plate but not as an electrode for applying a voltage. In sucha case, for example, the transmissive electrode 8 can be extended ontothe reflective electrode 3 to act as an electrode for applying a voltageto the liquid crystal layer 5 in the reflective region.

The liquid crystal material used herein has a refractive indexanisotropy of Ne (refractive index with respect to extraordinaryray)=1.5546, No (refractive index with respect to ordinary ray)=1,4773,and ΔN(Ne—No)=0.0773.

A λ/4 wave plate 7 is provided on the surface of the substrate 1opposite to the counter electrode 4. A slower optic axis of the λ/4 waveplate 7 is set to be tilted at 45 degrees with respect to a longitudinalaxis (i.e., molecular axis) of the liquid crystal molecules when avoltage is applied to the liquid crystal layer 5.

The λ/4 wave plate 7 is used for converting linearly polarized lightinto circularly polarized light and converting circularly polarizedlight into linearly polarized light.

The λ/4 wave plate 7 is provided on the surface of the substrate 1opposite to the counter electrode 4 in this example, but can be providedbetween the reflective electrode 3 and the liquid crystal layer 5.

The λ/4 wave plate 7 can be applied to the surface of the substrate 1 orintegrated with a polarizer 6 in order to reduce the production cost.

A polarizer 6 is provided on a surface of the λ/4 wave plate 7 oppositeto the substrate 1. A transmission axis of the polarizer 6 is set to betilted at 45 degrees with respect to the slower optic axis of the λ/4wave plate 7.

FIG. 7A is a plan view of an active matrix substrate (substrate 2) inthe first example, and FIG. 7B is a cross-sectional view of the activematrix substrate taken along line 7B-7B′ of FIG. 7A.

As shown in FIGS. 7A and 7B, the active matrix substrate includes a gateline 21, a data line 22, a driving element 23, a drain electrode 24, astorage capacitance electrode 25, a gate insulating layer 26, aninsulating substrate 27, a contact hole 28, an interlayer insulatinglayer 29 and a reflective electrode 30 (corresponding to the reflectiveelectrode 3 in FIG. 1).

The storage capacitance electrode 25 is electrically connected to thedrain electrode 24, and overlaps a storage capacitance line 32 with thegate insulating layer 26 being interposed therebetween. Thus, thestorage capacitance electrode 25, the insulating layer 26 and thestorage capacitance line 32 form a storage capacitance.

The contact hole 28 is formed in the interlayer insulating layer 29 forconnecting the reflective electrode 30 and the storage capacitanceelectrode 25.

With reference to FIGS. 13A and 13B, light transmission and reflectionin the LCD device in the reflection mode in the first example will bedescribed.

FIG. 13A shows black display performed when no voltage is applied to theliquid crystal layer 5, and FIG. 13B shows white display performed whena voltage is applied to the liquid crystal layer 5. In these figures,the reflective electrode 3 (reflective electrode region 3) is formed onthe left side.

With reference to FIG. 13A, black display will be described.

The light incident on the upper surface of the polarizer 6 istransmitted through the polarizer 6 to be linearly polarized lightparallel to the transmission axis of the polarizer 6 and then isincident on the λ/4 wave plate 7.

The λ/4 wave plate 7 is arranged so that the transmission axis of thepolarizer 6 and the slower optic axis of the λ/4 wave plate 7 make anangle of 45 degrees. Thus, the light transmitted through the λ/4 waveplate 7 becomes circularly polarized light.

When no voltage is applied to the liquid crystal layer 5, the liquidcrystal molecules exhibiting negative dielectric anisotropy used in theliquid crystal layer 5 are substantially vertical to the surfaces of thesubstrates 1 and 2. Accordingly, the refractive index anisotropy of theliquid crystal layer 5 to the incident light is very small. In otherwords, the phase difference caused by the transmission of the lightthrough the liquid crystal layer 5 is substantially zero.

Accordingly, the circularly polarized light from the λ/4 wave plate 7 istransmitted through the liquid crystal layer 5 while maintainingcircular polarization and reflected by the reflective electrode 3 in thesubstrate 2.

The circularly polarized light reflected by the reflective electrode 3is transmitted through the liquid crystal layer 5 toward the substrate 1and is incident on the λ/4 wave plate 7 as while maintaining circularpolarization.

Then, the circularly polarized light is transmitted through the λ/4 waveplate 7 to be linearly polarized light perpendicular to the transmissionaxis of the polarizer 6 and then is incident on the polarizer 6.

Since the polarization direction of the light is now perpendicular tothe transmission axis of the polarizer 6, the light is absorbed by thepolarizer 6 without being transmitted.

In this manner, black display is performed.

With reference to FIG. 13B, white display will be described.

The process until the light is transmitted through λ/4 wave plate 7 tobe circularly polarized light is the same as above and will not bedescribed.

When a voltage is applied to the liquid crystal layer 5, the liquidcrystal molecules are tilted horizontal with respect to the surfaces ofthe substrates 1 and 2. Accordingly, the circularly polarized lightincident on the liquid crystal layer 5 becomes elliptically polarizedlight by the birefringence of the liquid crystal molecules. The light isthen reflected by the reflective electrode 3, and the polarization ischanged after the light is transmitted through the liquid crystal layer5. After being transmitted through the λ/4 wave plate 7, the light doesnot become linearly polarized light perpendicular to the transmissionaxis of the polarizer 6. Thus, the light is transmitted through thepolarizer 6.

By controlling the voltage applied to the liquid crystal layer 5, theamount of light transmitted through the polarizer 6 after beingreflected by the reflective electrode 3 can be adjusted. Thus, grayscale display is provided.

When a voltage is applied to the liquid crystal layer 5 by thereflective electrode 3 and the counter electrode 4 to change theorientation of the liquid crystal molecules so that the phase differenceby the liquid crystal layer 5 fulfills the ¼ wavelength (λ/4) condition,the circularly polarized light from the λ/4 wave plate 7 becomeslinearly polarized light perpendicular to the transmission axis of thepolarizer 6 when reaching the reflective electrode 3 after beingtransmitted through the liquid crystal layer 5. The light is againtransmitted through the liquid crystal layer 5 and the λ/4 wave plate 7to be linearly polarized light parallel to the transmission axis of thepolarizer 6. In this case, the amount of light transmitted through thepolarizer 6 is maximum.

As described above, when no voltage is applied across the liquid crystallayer 5, black display is obtained since the liquid crystal layer 5 hassubstantially no birefringence; and when a voltage is applied across theliquid crystal layer 5, gray scale display is obtained by changing thelight transmittance in accordance with the voltage.

FIG. 4 shows the spectral reflectance characteristics of thereflection-type LCD device in the first example, which is obtained whenthe cell gap of the liquid crystal layer is d=3.56 μm and theretardation (phase difference) by the liquid crystal layer isdΔN=0.2752, in the case where the light is incident and receivedvertically.

In FIG. 4, the spectral reflectance to the single reflective plate inthe case where the light is incident and received vertically is 100.

As shown in FIG. 4, a sufficient contrast ratio of 50 or more isobtained over the entire wavelength range of 400 nm to 700 nm betweenthe black display when no voltage is applied and the white display whena voltage of 3.25 V is applied.

When a voltage of 3.25 V is applied, a reflectance of about 40% isobtained which is substantially equal to the transmittance of thepolarizer 6. Such a high light utilization factor is suitable for areflection-type LCD device.

FIG. 5 shows the spectral reflectance characteristics of thereflection-type LCD device in the first example, which is obtained whenthe cell gap of the liquid crystal layer is d=4.5 μm and the retardation(phase difference) by the liquid crystal layer is dΔN=0.3479, in thecase where the light is incident and received vertically.

As shown in FIG. 5, a sufficient contrast ratio of 50 or more isobtained in the entire wavelength range of 400 nm to 700 nm between theblack display when no voltage is applied and the white display when avoltage of 3 V is applied.

When a voltage of 3 V is applied, a reflectance of about 40% is obtainedas in the case where the cell gap d=3.56 μm.

FIG. 6 shows the relationship between the cell gap and the contrastratio of the reflection-type LCD device in the first example when thelight is incident and received vertically at a wavelength of 550 nm.

The contrast ratio is measured by applying a voltage by which theretardation (phase difference) dΔN by the liquid crystal layer fulfillsthe ¼ wavelength condition.

As shown in FIG. 6, the reflection-type LCD device in the first examplemaintains the contrast ratio of 500 or more regardless of the cell gapof the liquid crystal layer.

Accordingly, when a voltage is applied across the liquid crystal layer,display is provided without any reduction of contrast ratio as long asthe phase difference dΔN fulfills the ¼ wavelength condition. The cellgap d can be arbitrarily set.

FIG. 12 shows the relationship between the angle of the slower opticaxis of the λ/4 wave plate and the contrast ratio. The angle of theslower optic axis of the λ/4 wave plate is set to 0 degrees when theslower optic axis is tilted at 45 degrees with respect to thetransmission axis of the polarizer.

When the angular difference of the slower optic axis is within 3degrees, a contrast ratio of 500 or more is obtained, and thus areflection-type LCD device having satisfactory display characteristicsis provided.

In other words, high contrast is obtained even when the λ/4 wave plateand the polarizer are combined together with an angle of the sloweroptic axis of the λ/4 wave plate and the transmission axis of thepolarizer being slightly offset from the set value.

FIG. 6 shows the values after the influence of the reflection by thesurface of the panel is eliminated. In actual use, the reflection by thesurface of the panel is not negligible. The contrast ratio with thereflection by the panel being considered is about 20, which is stillsatisfactory for a reflection-type LCD device.

The LCD device in the first example using a vertically aligned liquidcrystal layer makes the retardation by the liquid crystal layersubstantially zero when no voltage is applied. In the case of normallyblack display, the darkness of the black state is improved, thusenhancing the contrast.

EXAMPLE 2

An LCD device in a second example according to the present inventionwill be described with reference to FIG. 2. Identical elements as thosein the first example bear identical reference numerals.

A substrate 2 includes a reflective electrode 3 (shown as reflectiveelectrode region in FIG. 2) formed of a material having a highreflectance such as, for example, Al or Ta and a transmissive electrode8 (shown as transmissive electrode region in FIG. 2) formed of amaterial having a high transmittance such as, for example, ITO. Asubstrate 1 includes a counter electrode 4 (shown as transmissiveelectrode in FIG. 2). A liquid crystal layer 5 formed of a liquidcrystal material exhibiting negative dielectric anisotropy is interposedbetween the reflective electrode 3/transmissive electrode 8 and thecounter electrode 4.

Alignment layers (not shown) are provided on surfaces of the reflectiveelectrode 3/transmissive electrode 8 and the counter electrode 4 whichare in contact with the liquid crystal layer 5. The alignment layers areused to align liquid crystal molecules (not shown) in the liquid crystallayer 5 vertically to surfaces of the substrates 1 and 2. After thealignment layers are provided, at least one of the alignment layers isprocessed with alignment treatment such as, for example, rubbing.

Due to the alignment treatment, the liquid crystal molecules in theliquid crystal layer 5 has a tilt angle of about 0.1 to 5 degrees withrespect to the vertical direction to the surfaces of the substrates 1and 2.

The reflective electrode 3 is used for applying a voltage to the liquidcrystal layer 5, but the reflective electrode 3 can be used only as areflective plate but not as an electrode for applying a voltage. In sucha case, for example, the transmissive electrode 8 can be extended ontothe reflective electrode 3 to act as an electrode for applying a voltageto the liquid crystal layer 5 in the reflective region.

The liquid crystal material used herein has a refractive indexanisotropy of Ne (refractive index with respect to extraordinaryray)=1.5546 and No (refractive index with respect to ordinaryray)=1.4773.

A λ/4 wave plate 7 is provided on the surface of the substrate 1opposite to the counter electrode 4. A slower optic axis of the λ/4 waveplate 7 is set to be tilted at 45 degrees with respect to a longitudinalaxis of the liquid crystal molecules when a voltage is applied acrossthe liquid crystal layer 5.

A λ/4 wave plate 10 is provided on the surface of the substrate 2opposite to the reflective electrode 3 and the transmissive electrode 8.A slower optic axis of the λ/4 wave plate 10 is parallel to the sloweroptic axis of the λ/4 wave plate 7.

A polarizer 6 is provided on a surface of the λ/4 wave plate 7 oppositeto the substrate 1. A polarizer 9 is provided on a surface of the λ/4wave plate 10 opposite to the substrate 2. A transmission axis of thepolarizers 6 is set to be tilted at 45 degrees with respect to theslower optic axis of the λ/4 wave plate 7. A transmission axis of thepolarizer 9 is set to be tilted at 45 degrees with respect to the sloweroptic axis of the λ/4 wave plate 10.

FIG. 8A is a plan view of an active matrix substrate (substrate 2) inthe second example, and FIG. 8B is a cross-sectional view of the activematrix substrate taken along line 8B-8B′ of FIG. 8A.

As shown in FIGS. 8A and 8B, the active matrix substrate includes a gateline 21, a data line 22, a driving element 23, a drain electrode 24, astorage capacitance electrode 35, a gate insulating layer 26, aninsulating substrate 27, a contact hole 28, an interlayer insulatinglayer 29, a reflective pixel electrode (reflective electrode region) 30(corresponding to the reflective electrode 3 in FIG. 2), and atransmissive pixel electrode (transmissive electrode region) 31(corresponding to the transmissive electrode 8 in FIG. 2).

The storage capacitance electrode 35 is electrically connected to thedrain electrode 24, and overlaps the gate line 21 with the gateinsulating layer 26 being interposed therebetween. Thus, the storagecapacitance electrode 35, the insulating layer 26 and the gate line 21form a storage capacitance.

The contact hole 28 is formed in the interlayer insulating layer 29 forconnecting the transmissive pixel electrode 31 and the storagecapacitance electrode 35.

The reflective pixel electrode 30 and the transmissive pixel electrode31 independently define a reflective region and a transmissive region ineach of a plurality of pixel areas in the LCD device. The reflectiveregion substantially performs reflection-mode display by reflectingexternal light, and the transmissive region substantially performstransmission-mode display by allowing the light from the backlight to betransmitted therethrough. Since there are, needless to say, lightcomponents incident on the LCD device obliquely in actual display, theboundary between the two region is not very clear.

As shown in FIG. 8A, it is typically preferable to provide thereflective electrode region 30 in a peripheral area of the pixel area,and to provide the transmissive electrode region 31 in a central area ofthe pixel area. By partially overlapping the reflective electrode region30 with the gate lines 21 and the data line 22, a storage capacitance isformed and the display area is enlarged.

In the second example, a semi-transmission- and semi-reflection-type LCDdevice is provided. FIG. 8C is a plan view of an active matrix substrateused in the semi-transmission- and semi-reflection-type LCD device. FIG.8D is a cross-sectional view of the active matrix substrate taken alongline 8D-8D′ of FIG. 8C, and FIG. 8E is a cross-sectional view of theactive matrix substrate taken along line 8E-8E′ of FIG. 8C.

The active matrix substrate shown in FIG. 8C includes a smalltransmissive electrode region 30T in the reflective electrode region 30.The reflective electrode region 30 and the transmissive electrode region30T do not independently define the reflective region and thetransmissive region, but the display in the reflection mode and thedisplay in the transmission mode are mixed and overlapped in the entirepixel area.

The active matrix substrate shown in FIG. 8C is produced by, forexample, as shown in FIG. 8D, forming a reflective electrode 30 having aplurality of openings 30T. Since a voltage is applied across the liquidcrystal molecules located on the openings 30T in the reflectiveelectrode 30 by the reflective electrode 30 (oblique electric fieldformed between the reflective electrode 30 and the counter electrode),formation of the transmissive electrode 31 can be omitted. In otherwords, the reflective electrode 30 can be formed of a semi-transmissiveand semi-reflective layer. Alternatively, when the reflective electrode30 is patterned by photolithography, openings having a prescribed shapecan be formed at a prescribed density. One dimension of the openingsshould not be larger than the thickness of the liquid crystal layer sothat a sufficient voltage can be applied across the liquid crystal layerby an oblique electric field. The electrode formed of asemi-transmissive and semi-reflective layer can be as described inJapanese Laid-Open Publication No. 7-333598. A semi-transmissive andsemi-reflective layer is formed by depositing metal particles to a verysmall thickness within a pixel area or formed by forming microscopicholes or recesses in a scattered manner within a pixel area.

Alternatively, the active matrix substrate shown in FIG. 8C is producedby, for example, as shown in FIG. 8E, forming a transmissive electrode44 on the reflective electrode region 30 having the openings in theentirety of the pixel area. In such a structure, the liquid crystalmolecules on the transmissive electrode region 30T are supplied with thesame level of voltage as the voltage applied across the liquid crystalmolecules on the reflective electrode region 30. During the etchingprocess for forming the transmissive electrode 44 (for example, when thereflective electrode region 30 is formed of Al and the transmissiveelectrode 44 is formed of ITO), electrocorrosion may occur between thereflective electrode region 30 and the transmissive electrode 44.Electrocorrosion is avoided by, as shown in FIG. 8E, forming aninterlayer insulating layer 42 (for example, formed of silicon oxide orpolymeric resin) on the reflective electrode region 30 and forming thetransmissive electrode 44 on the interlayer insulating layer 42.

With reference to FIGS. 13A and 13B, light transmission and reflectionin the LCD device in the transmission mode in the second example will bedescribed.

FIG. 13A shows black display performed when no voltage is applied acrossthe liquid crystal layer 5, and FIG. 13B shows white display performedwhen a voltage is applied across the liquid crystal layer 5. In thesefigures, the transmissive electrode 8 (transmissive electrode region) isformed on the right side. In FIGS. 13A and 13B, the reflective electroderegion 3 has the same structure as that in the first example and thuswill not be described. When the LCD device is used as thereflection-type LCD device, the LCD device operates in the same manneras in the first example. In the following description, the LCD device inthe transmission mode will be described and the LCD device in thereflection mode will not be repeated.

With reference to FIG. 13A, black display will be described.

The light emitted by a light source (not shown) is incident on thepolarizer 9 to be linearly polarized light parallel to the transmissionaxis of the polarizer 9.

The λ/4 wave plate 10 is arranged so that the slower optic axis thereofis tilted at 45 degrees with respect to the transmission axis of thepolarizer 9. Thus, the light transmitted through the λ/4 wave plate 10becomes circularly polarized light.

When no voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules exhibiting negative dielectric anisotropy used in theliquid crystal layer 5 are substantially vertical to the surfaces of thesubstrates 1 and 2. Accordingly, the refractive index anisotropy of theliquid crystal layer 5 to the incident light is very small. In otherwords, the phase difference caused by the transmission of the lightthrough the liquid crystal layer 5 is substantially zero.

Accordingly, the circularly polarized light from the λ/4 wave plate 10is transmitted through the liquid crystal layer 5 while maintainingcircular polarization and is incident on the λ/4 wave plate 7.

The slower optic axis of the λ/4 wave plate 10 and the slower optic axisof the λ/4 wave plate 7 are parallel to each other. Thus, the circularlypolarized light incident on the λ/4 wave plate 7 becomes linearlypolarized light perpendicular to the transmission axis of the polarizer9 and is incident on the polarizer 6.

The linearly polarized light from the λ/4 wave plate 7 is perpendicularto the transmission axis of the polarizer 6 and is absorbed by thepolarizer 6 without being transmitted.

In this manner, black display is performed.

With reference to FIG. 13B, white display will be described.

The process until the light is transmitted through λ/4 wave plate 10 isthe same as above and will not be described.

When a voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules are tilted horizontal with respect to the surfaces ofthe substrates 1 and 2. Accordingly, the circularly polarized lightincident on the liquid crystal layer 5 becomes elliptically polarizedlight by the birefringence of the liquid crystal molecules. The lightdoes not become linearly polarized light perpendicular to thetransmission axis of the polarizer 6 even after being transmittedthrough the λ/4 wave plate 7 and thus is transmitted through thepolarizer 6.

By controlling the voltage applied across the liquid crystal layer 5,the amount of light incident on the polarizer 6 can be adjusted. Thus,gray scale display is provided.

When a voltage is applied across the liquid crystal layer 5 so that thephase difference by the liquid crystal layer 5 fulfills the ½ wavelengthcondition, the circularly polarized light from the λ/4 wave plate 10becomes linearly polarized light perpendicular to the transmission axisof the polarizer 6 at the half of the thickness of the liquid crystallayer 5, and then becomes circularly polarized light when beingcompletely transmitted through the liquid crystal layer 5.

Since the circularly polarized light from the liquid crystal layer 5becomes linearly polarized light parallel to the transmission axis ofthe polarizer 6 when being transmitted through the λ/4 wave plate 7,most of the light incident on the polarizer 6 is transmittedtherethrough. In this case, the amount of light transmitted through thepolarizer 6 is maximum.

As described above, when no voltage is applied across the liquid crystallayer 5, black display is obtained both in the reflective electroderegion 3 and the transmissive electrode region 8 since there is nobirefringence of the liquid crystal layer 5. When a voltage is appliedacross the liquid crystal layer 5 while controlling the level of thevoltage, the amount of light transmitted through the LCD device isadjusted and thus gray scale display is obtained.

FIG. 9 shows the spectral reflectance characteristics of thetransmission- and reflection-type LCD device in the second example,which is obtained when the cell gap of the liquid crystal layer isd=3.56 μm and the phase difference by the liquid crystal layer isdΔN=0.2752, in the case where the light is incident and receivedvertically.

In FIG. 9, the spectral reflectance in the reflective electrode regionis the same as in FIG. 4.

In FIG. 9, the spectral reflectance to the air in the case where thelight is incident and received vertically is 100.

As shown in FIG. 9, sufficient contrast is obtained in the entirewavelength range of 400 nm to 700 nm between the black display when novoltage is applied and the white display when a voltage of 5 V isapplied.

When a voltage of 5 V is applied, a reflectance of about 30% is obtainedwhich is about 80% of the transmittance of the polarizer 6. Such a highlight utilization factor is suitable for a transmission- andreflection-type LCD device.

FIG. 10 shows the spectral reflectance characteristics of thetransmission- and reflection-type LCD device in the second example,which is obtained when the cell gap of the liquid crystal layer is d=4.5μm and the phase difference by the liquid crystal layer is dΔN=0.3749,in the case where the light is incident and received vertically.

As shown in FIG. 10, a sufficient contrast ratio is obtained in theentire wavelength range of 400 nm to 700 nm between the black displaywhen no voltage is applied and the white display when a voltage of 5 Vis applied.

When a voltage of 5 V is applied, a reflectance of about 40% isobtained.

FIG. 11 shows the relationship between the cell gap and the contrastratio of the transmission- and reflection-type LCD device in the secondexample when the light is incident and received vertically at awavelength of 550 nm.

The contrast ratio is measured by applying a voltage by which the phasedifference dΔN by the liquid crystal layer fulfills the ½ wavelengthcondition.

As shown in FIG. 11, the transmission- and reflection-type LCD device inthe second example maintains the contrast ratio of 800 or more in thetransmissive electrode region (used as a transmission-type LCD device)and maintains the contrast ratio of 500 or more in the reflectiveelectrode region (used as a reflection-type LCD device) regardless ofthe cell gap of the liquid crystal layer.

Accordingly, when a voltage is applied across the liquid crystal layer,display is provided without any reduction of contrast ratio as long asthe phase difference dΔN fulfills the ½ wavelength condition. The cellgap d can be arbitrarily set.

FIG. 12 shows the relationship between the angle of the slower opticaxis of the λ/4 wave plate and the contrast ratio. The angle of theslower optic axis of the λ/4 wave plate is set to 0 degrees when theslower optic axis is tilted at 45 degrees with respect to thetransmission axis of the polarizer.

When the angular difference of the slower optic axis is within 3degrees, a contrast ratio of 50 or more is obtained both in thetransmissive electrode region (when the LCD device is used as atransmission-type LCD device) and in the reflective electrode region(when the LCD device is used as a reflection-type LCD device), and thusa transmission- and reflection-type LCD device having satisfactorydisplay characteristics is provided.

Accordingly, one LCD device can be used both as a transmission-type LCDdevice for performing display using the light from the backlighttransmitted through the transmissive electrode 8 when the ambient lightis dark, and as a reflection-type LCD device for performing displayusing the ambient light reflected by the reflective electrode 3 formedof a material having a relatively high light reflectance when theambient light is bright. Moreover, the LCD device can use both thebacklight and the ambient light.

When the ambient light is bright, the backlight need not be used. Thus,the power consumption is reduced compared to the conventionaltransmission-type LCD device. When the ambient light is dark, thebacklight can be used. Thus, the problem of the conventionalreflection-type LCD device that the sufficient display is not obtainedis overcome.

The LCD device in the second example using a vertically aligned liquidcrystal layer makes the retardation by the liquid crystal layersubstantially zero when no voltage is applied. In the case of normallyblack display, the darkness of the black state both in the transmissionmode and the reflection mode is improved, thus enhancing the contrast.

EXAMPLE 3

An LCD device in a third example according to the present invention willbe described with reference to FIG. 3. Identical elements as those inthe first and second examples bear identical reference numerals anddetailed descriptions thereof will be omitted.

The LCD device in the third example includes a λ/4 wave plate 10 and aphase compensation element 12 between the substrate 2 and the polarizer9 and also includes λ/4 wave plate 7 and a phase compensation element 11between the substrate 1 and the polarizer 6.

The positions of the λ/4 wave plate 10 and the phase compensationelement 12, and the positions of the λ/4 wave plate 7 and the phasecompensation element 11 are exchangeable, respectively.

When no voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules of the liquid crystal material exhibiting negativedielectric anisotropy in the liquid crystal layer 5 are alignedsubstantially vertically to the surfaces of the substrates 1 and 2.Thus, the refractive index anisotropy of the liquid crystal layer 5 tothe light incident on the LCD device vertically is substantially nil.

When the LCD device is used as a reflection-type LCD device, however,light incident in other directions as well as light incident verticallyis used for display. When the light incident on the liquid crystal layer5 obliquely including ambient light is used for display, the display isinfluenced by the refractive index anisotropy.

The viewing angle is not necessarily vertical to the surface of thesubstrates. As the viewing angle is offset from the vertical directionto the surface of the substrates, the display is more influenced by therefractive index anisotropy of the liquid crystal molecules in theliquid crystal layer 5. Thus, the contrast ratio is reduced.

In this example, the phase compensation elements 11 and 12 forcompensating for the influence by such refractive index anisotropy ofthe liquid crystal molecules are provided to prevent the contrast frombeing reduced in accordance with the incident angle of light and theviewing direction.

In the case where the pretilt angle of the liquid crystal molecules isslightly tilted with respect to the vertical direction to the surface ofthe substrates so that the liquid crystal molecules are tilted in onedirection when a voltage is applied across the vertically aligned liquidcrystal layer 5, slight refractive index anisotropy is caused in thevertical direction to the substrates even when no voltage is applied.The phase compensation elements are also used to compensate for therefractive index anisotropy and thus to further improve the contrast inthe vertical direction to the substrates.

In this example, the λ/4 wave plate and the phase compensation elementare described as being separate, but the same effects are obtained whenthe λ/4 wave plate and the phase compensation element are in the samelayer.

In this example, two phase compensation elements 11 and 12 are provided,but only one phase compensation element 11 can be satisfactory.

In the third example, the transmission- and reflection-type LCD deviceis described. In the case of the reflection-type LCD device in the firstexample also (FIG. 1), a phase compensation element can be providedbetween the polarizer 6 and the reflective electrode 3 to compensate forthe refractive index anisotropy of the liquid crystal layer 5. Thus,reduction in the contrast is prevented.

In the first through third example, black display and white display aredescribed. Color display is also realized by providing a color filter onappropriate areas of the reflective electrode region and thetransmissive electrode region.

When a chiral dopant is added to the vertically aligned liquid crystallayer formed of a liquid crystal material exhibiting negative dielectricanisotropy of the LCD devices in the first through third examples, theliquid crystal molecules are rotated when a voltage is applied. Thus,the rotation of the liquid crystal molecules at the time of voltageapplication is stabilized.

When the liquid crystal layer is aligned to have a 90 degree twist,black display with very little optical leakage is obtained for thefollowing reason. When the liquid crystal molecules are aligned to tiltat several degrees with respect to the normal direction to the surfacesof the substrates in order to prevent disclination when a voltage isapplied, retardation is caused in the tilting direction of the liquidcrystal molecules. However, since the liquid crystal molecules in areasin the vicinity of the top and bottom substrates make an angle of 90degrees, the retardation is counteracted. Thus, the resultant, blackdisplay has very little optical leakage.

The LCD devices in the first through third examples use a verticallyaligned liquid crystal: layer formed of a material having negativedielectric anisotropy. The same effects are obtained when the liquidcrystal layer is treated so that the liquid crystal molecules arealigned horizontal to the surface of the substrates.

In such a case, the liquid crystal molecules are aligned horizontal tothe surfaces when no voltage is applied, and the liquid crystalmolecules tilt toward the normal direction to the substrates when avoltage is applied. Accordingly, white display is performed when novoltage is applied, and black display is performed when a voltage isapplied.

In the case of the black display by the horizontally aligned liquidcrystal layer, the remaining retardation is larger than in the case ofthe vertically aligned liquid crystal layer due to the liquid crystalmolecules in the vicinity of the substrates. In order to perform morecomplete black display, a phase compensation element can be used.

In the case where the liquid crystal molecules are aligned almostvertically to the substrates and the retardation of α is remaining inthe reflection mode, a phase compensation element can be used having aretardation of λ/4-a in lieu of the λ/4 wave plate 7 (FIGS. 1, 2 and 3).

In the reflection mode, elliptically polarized light which is offsetfrom the circularly polarized light by the remaining retardation of theliquid crystal layer is incident on the liquid crystal layer. Theelliptically polarized light becomes circularly polarized light whenreaching the reflective electrode region after being transmitted throughthe liquid crystal layer. As a result of the reflection, the lightbecomes circularly polarized light having an opposite rotationdirection. The light becomes elliptically polarized light offset fromthe circularly polarized light when being transmitted through and goingout of the liquid crystal layer. The elliptically polarized light atthis point is phase-offset at 90 degrees from the light which wasincident. When being transmitted through the phase compensation element,the elliptically polarized light becomes linearly polarized lightperpendicular to the transmission axis of the polarizer 6.

As can be appreciated, even when the retardation remaining in thevertically aligned liquid crystal layer is not negligible, high contrastdisplay is obtained in the reflection mode by providing a phasecompensation element in consideration of the retardation.

An LCD device shown in FIG. 2 including a horizontally aligned liquidcrystal layer 5 will be described.

The liquid crystal layer 5 is formed of a material available from Merck& Co., Inc. and has Ne=1.5328, No=1.4722 and ΔN=0.0606. The thickness ofthe liquid crystal layer 5 in the transmissive region is about 5.2 μm.

The alignment layers provided on the substrates 1 and 2 are treated byrubbing in the direction perpendicular to the gate line (or sourceline). The substrates 1 and 2 are combined so that the alignment layerson the substrates 1 and 2 are opposite to each other (anti-parallel).When no voltage is applied across the liquid crystal layer 5, themolecular axis of the liquid crystal molecules in the liquid crystallayer 5 is aligned parallel to the surfaces of the substrates 1 and 2and perpendicular to the gate line. When a voltage is applied, themolecular axis of the liquid crystal molecules tilts in the normaldirection to the surfaces of the substrates 1 and 2 while beingsubstantially perpendicular to the gate line. In this example, the axesof the polarizers 6 and 9 and the phase compensation elements 7 and 10are set with the conditions that the voltage to be applied across theliquid crystal layer 5 for white display is about 1.8 V and the voltageto be applied across the liquid crystal layer 5 for black display isabout 5.3 V.

The transmission axis of the polarizer 6 is set to be about 45 degreesclockwise with respect to the molecular axis of the liquid crystalmolecules, and the slower optic axis of the phase compensation element 7is set to be about 45 degrees clockwise with respect to the transmissionaxis of the polarizer 6. In other words, the slower optic axis of thephase compensation element 7 is set to be about 90 degrees clockwisewith respect to the molecular axis of the liquid crystal molecules.

In consideration of the retardation by the liquid crystal layer 5 in thereflective region in the black display, two types of phase compensationelements having a retardation of about 105 nm and a retardation of about95 nm are used. Such retardations are offset from the λ/4 condition(about 137.5 nm). By using a phase compensation element having aretardation of λ/4-α, satisfactory contrast is realized in thereflective region.

The slower optic axis of the phase compensation element 10 and thetransmission axis of the polarizer 9 are set in consideration of theretardation by the liquid crystal layer 5 in the transmission region inthe black display. For a transmission- and reflection-type LCD device,first, the orientation of the polarizer 6 and the retardation and sloweroptic axis of the phase compensation element 7 are determined withrespect to the reflection region. Then, the retardation and slowerorientation of the polarizer 6 and the retardation and slower optic axisof the phase compensation element 10 and the orientation of thepolarizer 9 are determined with respect to the transmission region. Achange in the polarization state of the light transmitted through eachlayer in the transmissive region is equivalent when the light isincident on the display surface and when the light is incident from thebacklight. The following description will be given regarding the lightincident on the display surface for better understanding.

The linearly polarized light incident on the liquid crystal layer 5 inthe black state through the polarizer 6 goes out of the liquid crystallayer 6 as elliptically polarized light having a longer axis or shorteraxis at 45 degrees clockwise with respect to the molecular axis of theliquid crystal molecules. The elliptically polarized light can beconverted into linearly polarized light by providing the phasecompensation element 10 formed of a λ/4 wave plate having a retardationof about 140 nm and locating the slower optic axis thereof in the samedirection as the longer axis of the elliptically polarized light goingout of the liquid crystal layer 5, i.e., at 45 degrees clockwise withrespect to the molecular axis of the liquid crystal molecules. Then, thepolarizer 9 is located so that the transmission axis thereof isperpendicular to the polarizing axis of the linearly polarized lightgoing out of the phase compensation element 10.

The angle of the polarizing axis of the linearly polarized light goingout of the phase compensation element 10 depends on the polarizationstate of the elliptically polarized light incident on the phasecompensation element 10. In this example, when the retardation of thephase compensation element 7 is about 105 nm, the polarizing axis of thelinearly polarized light going out of the phase compensation element 10is at about 10 degrees clockwise with respect to the molecular axis ofthe liquid crystal molecules. Accordingly, by setting the transmissionaxis of the polarizer 9 at about 10 degrees clockwise with respect tothe molecular axis of the liquid crystal molecules, satisfactory blackdisplay is obtained in the transmission region. When the retardation ofthe phase compensation element 7 is about 95 nm, the polarizing axis ofthe linearly polarized light going out of the phase compensation element10 is at about 97 degrees clockwise with respect to the molecular axisof the liquid crystal molecules. Accordingly, by setting thetransmission axis of the polarizer 9 at about 7 degrees clockwise withrespect to the molecular axis of the liquid crystal molecules,satisfactory black display is obtained in the transmission region.

An LCD device shown in FIG. 3 including a horizontally aligned liquidcrystal layer 5 will be described.

The liquid crystal layer 5 is formed of a material available from Merck& Co., Inc. and has Ne=1.5328, No=1.4722 and ΔN=0.0606. The thickness ofthe liquid crystal layer 5 is about 5.2 μm. The alignment layers arepositioned to be anti-parallel. The axes of the polarizers 6 and 9 andthe phase compensation elements 7, 10, 11 and 12 are set with theconditions that the voltage to be applied across the liquid crystallayer 5 for white display is about 1.8 V and the voltage to be appliedacross the liquid crystal layer 5 for black display is about 5.3 V.

The transmission axis of the polarizer 6 is set to be about 15 degreesclockwise with respect to the molecular axis of the liquid crystalmolecules. The phase compensation element 11 is formed of a λ/2 waveplate having a retardation of about 270 nm. The phase compensationelement 11 is positioned so that the slower optic axis thereof is atabout 30 degrees clockwise with respect to the molecular axis of theliquid crystal molecules. Furthermore, the phase compensation element 7is positioned so that the slower optic axis thereof is at about 90degrees clockwise with respect to the molecular axis of the liquidcrystal molecules. The positioning is performed from the order of thepolarizer 6, the phase compensation element 11 and the phasecompensation element 7. In consideration of the retardation by theliquid crystal layer 5 in the reflective region in the black display,two types of phase compensation elements having a retardation of about105 nm and a retardation of about 95 nm are used. Such retardations areoffset from the λ/4 condition (about 137.5 nm). By using a phasecompensation element having a retardation of λ/4-α, satisfactorycontrast is realized in the reflective region.

The slower optic axis of the phase compensation elements 10 and 12 andthe transmission axis of the polarizer 9 are set in consideration of theretardation by the liquid crystal layer 5 in the transmission region inthe black display. The positioning is performed from the order of thephase compensation element 10, the phase compensation element 12, andthe polarizer 9.

The linearly polarized light incident on the liquid crystal layer 5 inthe black state through the polarizer 6 goes out of the liquid crystallayer 6 as elliptically polarized light having a longer axis or shorteraxis at 45 degrees clockwise with respect to the molecular axis of theliquid crystal molecules. The elliptically polarized light can beconverted into linearly polarized light by providing the phasecompensation element 10 formed of a λ/4 wave plate having a retardationof about 140 nm and locating the slower optic axis thereof in the samedirection as the longer axis of the elliptically polarized light goingout of the liquid crystal layer 5, i.e., at 45 degrees clockwise withrespect to the molecular axis of the liquid crystal molecules. Then, thephase compensation element 12 formed of a λ/2 wave plate having aretardation of about 270 nm is located so that the slower optic axisthereof is at 114 degrees clockwise with respect to the molecular axisof the liquid crystal molecules. Then, the polarizer 9 is located sothat the transmission axis thereof is perpendicular to the polarizingaxis of the linearly polarized light going out of the phase compensationelement 12.

The angle of the polarizing axis of the linearly polarized light goingout of the phase compensation element 12 depends on the polarizationstate of the elliptically polarized light incident on the phasecompensation element 10. In this example, when the retardation of thephase compensation element 7 is about 105 nm, the polarizing axis of thelinearly polarized light going out of the phase compensation element 10is at about 10 degrees clockwise with respect to the molecular axis ofthe liquid crystal molecules. The polarizing axis of the linearlypolarized light going out of the phase compensation element 12 is atabout 128 degrees clockwise with respect to the molecular axis of theliquid crystal molecules. Accordingly, by setting the transmission axisof the polarizer 9 at about 38 degrees clockwise with respect to themolecular axis of the liquid crystal molecules, satisfactory blackdisplay is obtained in the transmission region. When the retardation ofthe phase compensation element 7 is about 95 nm, the polarizing axis ofthe linearly polarized light going out of the phase compensation element10 is at about 97 degrees clockwise with respect to the molecular axisof the liquid crystal molecules. The polarizing axis of the linearlypolarized light going out of the phase compensation element 12 is atabout 125 degrees clockwise with respect to the molecular axis of theliquid crystal molecules. Accordingly, by setting the transmission axisof the polarizer 9 at about 35 degrees clockwise with respect to themolecular axis of the liquid crystal molecules, satisfactory blackdisplay is obtained in the transmission region.

In the case where the liquid crystal layer has a remaining retardationof α in the reflection mode and a remaining retardation of β in thetransmission mode, a phase compensation element having a retardation ofλ/4-α can be provided in lieu of the λ/4 wave plate 7 and a phasecompensation element having a retardation of λ/4-(β-α) can be providedin lieu of the λ/4 wave plate 10.

In the transmission mode using the light transmitted through a regionhaving a transmission function such as the transmissive electroderegion, when the liquid crystal molecules are aligned vertically to thesubstrates, a phase compensation element having a retardation ofλ/4-(β-α) is set so that the light going out of the liquid crystal layeris elliptically polarized light in the same state as in the reflectionmode. The elliptically polarized light having such a phase difference isincident on the phase compensation element having a retardation ofλ/4-α. Thus, when being transmitted through the phase compensationelement having a retardation of λ/4-α, the light becomes linearlypolarized light perpendicular to the transmission axis of the polarizer6. Accordingly, black display with very little optical leakage isobtained.

As can be appreciated, even when the retardation remaining in thevertically aligned liquid crystal layer is not negligible, high contrastdisplay is obtained in the reflection mode by providing a phasecompensation element in consideration of the retardation.

EXAMPLE 4

An LCD device in a fourth example according to the present inventionwill be described with reference to FIG. 2. Identical elements as thosein the first example bear identical reference numerals.

A substrate 2 includes a reflective electrode 3 (shown as reflectiveelectrode region in FIG. 2) formed of a material having a highreflectance such as, for example, Al or Ta and a transmissive electrode8 (shown as transmissive electrode region in FIG. 2) formed of amaterial having a high transmittance such as, for example, ITO. Asubstrate 1 includes a counter electrode 4 (shown as transmissiveelectrode in FIG. 2). A liquid crystal layer 5 formed of a liquidcrystal material exhibiting negative dielectric anisotropy is interposedbetween the reflective electrode 3/transmissive electrode 8 and thecounter electrode 4.

Alignment layers (not shown) are provided on surfaces of the reflectiveelectrode 3/transmissive electrode 8 and the counter electrode 4 whichare in contact with the liquid crystal layer 5. The alignment layers areused to align liquid crystal molecules (not shown) in the liquid crystallayer 5 vertically to surfaces of the substrates 1 and 2. After thealignment layers are provided, at least one of the alignment layers isprocessed with alignment treatment such as, for example, rubbing. Thealignment direction can be defined by optical alignment or electrodeshape in lieu of rubbing.

Due to the alignment treatment, the liquid crystal molecules in theliquid crystal layer 5 have a tilt angle of about 0.1 to 5 degrees withrespect to the vertical direction to the surfaces of the substrates 1and 2.

The reflective electrode 3 is used for applying a voltage to the liquidcrystal layer 5, but the reflective electrode 3 can be used only as areflective plate but not as an electrode for applying a voltage. In sucha case, for example, the transmissive electrode 8 can be extended ontothe reflective electrode 3 to act as an electrode for applying a voltageto the liquid crystal layer 5 in the reflective region.

The liquid crystal material used herein has a refractive indexanisotropy of Ne (refractive index with respect to extraordinaryray)=1.5546 and No (refractive index with respect to ordinaryray)=1.4773.

A λ/4 wave plate 7 is provided on the surface of the substrate 1opposite to the counter electrode 4. A λ/4 wave plate 10 is provided onthe surface of the substrate 2 opposite to the reflective electrode 3and the transmissive electrode 8. A slower optic axis of the λ/4 waveplate 10 is set to be perpendicular to the slower optic axis of the λ/4wave plate 7.

A polarizer 6 is provided on a surface of the λ/4 wave plate 7 oppositeto the substrate 1. A polarizer 9 is provided on a surface of the λ/4wave plate 10 opposite to the substrate 2. A transmission axis of thepolarizers 6 is set to be tilted at 45 degrees with respect to theslower optic axis of the λ/4 wave plate 7. A transmission axis of thepolarizer 9 is set to be tilted at 45 degrees with respect to the sloweroptic axis of the λ/4 wave plate 10. The slower optic axes of the λ/4wave plates 7 and 10 are perpendicular to each other and thetransmission axes of the polarizers 6 and 9 are perpendicular to eachother. Therefore, when the angle of the slower optic axis of the phasecompensation element 7 with respect to the transmission axis of thepolarizer 6 is +45 degrees, the angle of the slower optic axis of thephase compensation element 10 with respect to the transmission axis ofthe polarizer 9 is also +45 degrees. When the angle of the slower opticaxis of the phase compensation element 7 with respect to thetransmission axis of the polarizer 6 is −45 degrees, the angle of theslower optic axis of the phase compensation element 10 with respect tothe transmission axis of the polarizer 9 is also −45 degrees.

FIG. 8A is a plan view of an active matrix substrate (substrate 2) inthe fourth example, and FIG. 8B is a cross-sectional view of the activematrix substrate taken along line 8B-8B′ of FIG. 8A.

As shown in FIGS. 8A and 8B, the active matrix substrate includes a gateline 21, a data line 22, a driving element 23, a drain electrode 24, astorage capacitance electrode 25, a gate insulating layer 26, aninsulating substrate 27, a contact hole 28, an interlayer insulatinglayer 29, a reflective pixel electrode (reflective electrode region) 30(corresponding to the reflective electrode 3 in FIG. 2), and atransmissive pixel electrode (transmissive electrode region) 31(corresponding to the transmissive electrode 8 in FIG. 2).

The storage capacitance electrode 25 is electrically connected to thedrain electrode 24, and overlaps the gate line 21 with the gateinsulating layer 26 being interposed therebetween. Thus, the storagecapacitance electrode 25, the insulating layer 26 and the gate line 21form a storage capacitance.

The contact hole 28 is formed in the interlayer insulating layer 29 forconnecting the transmissive pixel electrode 31 and the storagecapacitance electrode 25.

The active matrix substrate includes a reflective pixel electrode 30 forreflecting the external light and a transmissive pixel electrode 31 forallowing the light from the backlight to be transmitted therethrough inone pixel area.

In FIG. 8B, the reflective electrode 30 has a flat surface, but can havea wave-like surface in order to improve the reflectance. One pixelelectrode includes the reflective pixel electrode 30 and thetransmissive pixel electrode 31 in this example. Alternatively, asemi-transmissive and semi-reflective electrode is usable.

With reference to FIGS. 14A, 14B, 15A and 15B, light transmittance andreflectance in the transmission mode and the reflectance mode of the LCDdevice in the fourth example will be described.

FIGS. 14A and 14B show the reflection mode using the reflectiveelectrode. FIG. 14A shows the black display when no voltage is appliedacross the vertically aligned liquid crystal layer, and FIG. 14B showsthe white display when a voltage is applied across the verticallyaligned liquid crystal layer. FIGS. 15A and 15B show the transmissionmode using the transmissive electrode. FIG. 15A shows the black displaywhen no voltage is applied across the vertically aligned liquid crystallayer, and FIG. 15B shows the white display when a voltage is appliedacross the vertically aligned liquid crystal layer.

With reference to FIG. 14A, black display in the reflection mode will bedescribed.

The light incident on the polarizer 6 is transmitted through thepolarizer 6 to be linearly polarized light parallel to the transmissionaxis of the polarizer 6 and then is incident on the λ/4 wave plate 7.

The λ/4 wave plate 7 is arranged so that the transmission axis of thepolarizer 6 and the slower optic axis of the λ/4 wave plate 7 make anangle of 45 degrees. Thus, the light transmitted through the λ/4 waveplate 7 becomes circularly polarized light.

When no voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules exhibiting negative dielectric anisotropy used in theliquid crystal layer 5 are substantially vertical to the surfaces of thesubstrates 1 and 2. Accordingly, the refractive index anisotropy of theliquid crystal layer 5 to the incident light is very small. In otherwords, the phase difference caused by the transmission of the lightthrough the liquid crystal layer 5 is substantially zero.

Accordingly, the circularly polarized light from the λ/4 wave plate 7 istransmitted through the liquid crystal layer 5 as almost beingcircularly polarized and reflected by the reflective electrode 3 in thesubstrate 2.

The circularly polarized light reflected by the reflective electrode 3becomes circularly polarized light having an opposite rotationdirection, and is transmitted through the λ/4 wave plate 7 to belinearly polarized light perpendicular to the light which was incidenton the λ/4 wave plate 7 from the polarizer 6.

The linearly polarized light from the λ/4 wave plate 7 is perpendicularto the transmission axis of the polarizer 6. Such light is absorbed bythe polarizer 6 without being transmitted.

In this manner, black display is performed.

With reference to FIG. 14B, white display in the reflection mode will bedescribed.

The process until the light is transmitted through λ/4 wave plate 7 tobe circularly polarized light is the same as above and will not bedescribed.

When a voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules are slightly tilted toward the horizontal directionwith respect to the surfaces of the substrates 1 and 2. Accordingly, thecircularly polarized light incident on the liquid crystal layer 5 fromthe λ/4 wave plate 7 becomes elliptically polarized light by thebirefringence of the liquid crystal molecules. The light is thenreflected by the reflective electrode 3, and further influenced by thebirefringence of the liquid crystal molecules in the liquid crystallayer 5. After being transmitted through the λ/4 wave plate 7, the lightdoes not become linearly polarized light perpendicular to thetransmission axis of the polarizer 6. Thus, the light is transmittedthrough the polarizer 6.

By controlling the voltage applied across the liquid crystal layer 5,the amount of light transmitted through the polarizer 6 after beingreflected by the reflective electrode 3 can be adjusted. Thus, grayscale display is provided.

When a voltage is applied across the liquid crystal layer 5 by thereflective electrode 3 and the counter electrode 4 to change thealignment of the liquid crystal molecules so that the phase differenceby the liquid crystal layer 5 fulfills the ¼ wavelength condition, thecircularly polarized light from the λ/4 wave plate 7 becomes linearlypolarized light perpendicular to the transmission axis of the polarizer6 when reaching the reflective electrode 3 after being transmittedthrough the liquid crystal layer 5. The light is again transmittedthrough the liquid crystal layer 5 to be circularly polarized light andthen transmitted through the λ/4 wave plate 7 to be linearly polarizedlight parallel to the transmission axis of the polarizer 6. In thiscase, the amount of light transmitted through the polarizer 6 ismaximum.

FIG. 14B shows the case having the liquid crystal layer retardationconditions by which a maximum amount of light reflected by thereflective electrode 3 is transmitted through the polarizer 6. In otherwords, the light on the reflective electrode 3 is linearly polarizedlight perpendicular to the transmission axis of the polarizer 6.

As described above, when no voltage is applied across the liquid crystallayer 5, black display is obtained since the liquid crystal layer 5 hassubstantially no birefringence; and when a voltage is applied across theliquid crystal layer 5, gray scale display is obtained by changing thelight transmittance in accordance with the voltage.

With reference to FIG. 15A, black display in the transmission mode willbe described.

The light emitted by a light source (not shown) is incident on thepolarizer 9 to be linearly polarized light parallel to the transmissionaxis of the polarizer 9.

The λ/4 wave plate 10 is arranged so that the slower optic axis thereofis tilted at 45 degrees with respect to the transmission axis of thepolarizer 9. Thus, the light transmitted through the λ/4 wave plate 10is circularly polarized light.

When no voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules exhibiting negative dielectric anisotropy used in theliquid crystal layer 5 are substantially vertical to the surfaces of thesubstrates 1 and 2. Accordingly, the refractive index anisotropy of theliquid crystal layer 5 to the incident light is very small. In otherwords, the phase difference caused by the transmission of the lightthrough the liquid crystal layer 5 is substantially zero.

Accordingly, the circularly polarized light from the λ/4 wave plate 10is transmitted through the liquid crystal layer 5 while maintainingcircular polarization and is incident on the λ/4 wave plate 7.

The slower optic axis of the λ/4 wave plate 10 and the slower optic axisof the λ/4 wave plate 7 are set to be perpendicular to each other. Thus,the circularly polarized light incident on the λ/4 wave plate 7 becomeslinearly polarized light perpendicular to the transmission axis of thepolarizer 9 and is incident on the polarizer 6.

The linearly polarized light from the λ/4 wave plate 7 is perpendicularto the transmission axis of the polarizer 6 and is absorbed by thepolarizer 6 without being transmitted.

In this manner, black display is performed.

With reference to FIG. 15B, white display in the transmission mode willbe described.

The process until the light is transmitted through λ/4 wave plate 10 isthe same as in FIG. 15A and will not be described.

When a voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules are slightly tilted toward the horizontal directionwith respect to the surfaces of the substrates 1 and 2. Accordingly, thecircularly polarized light incident on the liquid crystal layer 5 fromthe λ/4 wave plate 10 becomes elliptically polarized light by thebirefringence of the liquid crystal molecules. The light does not becomelinearly polarized light perpendicular to the transmission axis of thepolarizer 6 even after being transmitted through the λ/4 wave plate 7and thus is transmitted through the polarizer 6.

By controlling the voltage applied across the liquid crystal layer 5,the amount of light transmitted through the polarizer 6 can be adjusted.Thus, gray scale display is provided.

When a voltage is applied across the liquid crystal layer 5 so that thephase difference by the liquid crystal layer 5 fulfills the ½ wavelengthcondition, the circularly polarized light from the λ/4 wave plate 10becomes linearly polarized light perpendicular to the transmission axisof the polarizer 6 at the half of the thickness of the liquid crystallayer 5, and then becomes circularly polarized light when beingcompletely transmitted through the liquid crystal layer 5.

Since the circularly polarized light from the liquid crystal layer 5becomes linearly polarized light parallel to the transmission axis ofthe polarizer 6 when being transmitted through the λ/4 wave plate 7,most of the light incident on the polarizer 6 is transmittedtherethrough. In this case, the amount of light transmitted through thepolarizer 6 is maximum.

FIG. 15B shows the case having the liquid crystal layer retardationconditions by which a maximum amount of light transmitted through thepolarizer 9 is transmitted through the polarizer 6.

As described above, when no voltage is applied across the liquid crystallayer 5, black display is obtained; and when a voltage is applied acrossthe liquid crystal layer 5, gray scale display is obtained by changingthe light transmittance in accordance with the voltage.

FIG. 16 shows the relationship between the wavelength and thetransmittance of the light when the slower optic axes of the λ/4 waveplates 7 and 10 are perpendicular to each other as in the fourth exampleand when the slower optic axes of the λ/4 wave plates 7 and 10 areparallel to each other for comparison.

In the fourth example, since the slower optic axes of the λ/4 waveplates 7 and 10 are perpendicular to each other, the wavelengthdependency of the refractive index anisotropy of one phase compensationelement is counteracted by the wavelength dependency of the refractiveindex anisotropy of the other phase compensation element. Thus, aprescribed phase difference is fulfilled in the entire region of thewavelengths from 400 nm to 700 nm (visible light). Thus, the darkness ofthe black display is improved.

The phase difference by the liquid crystal layer 5 at which thereflectance is maximum in the bright display in the reflection mode isλ/4, and the phase difference by the liquid crystal layer 5 at which thereflectance is maximum in the bright display in the transmission mode isλ/2. As is appreciated from this, when the thickness of the liquidcrystal layer in the reflective region and the thickness of the liquidcrystal in the transmissive region are equal to each other, the phasedifferences of λ/4 for the reflection mode and λ/4 for the transmissionmode cannot be fulfilled at the same time.

In the case where display is performed by changing the phase differenceof the liquid crystal layer in the reflective region from 0 to λ/4, asatisfactory light utilization factor cannot be obtained in thetransmission mode since the phase difference of the liquid crystal layerin the transmissive region also changes only from 0 to λ/4.

Satisfactory light utilization factors both in the reflection mode andthe transmission mode are achieved by changing the thickness of theliquid crystal layer in the reflective region from the thickness in thetransmissive region, or by applying different voltages to the liquidcrystal layer in the reflection region and to the liquid crystal layerin the transmissive region. In the case where the thickness of theliquid crystal layer in the transmission region is made twice thethickness of the liquid crystal layer in the reflective region, thephase differences of the liquid crystal layer of λ/4 for the reflectionmode and of λ/2 for the transmission mode are fulfilled at the sametime. It is not necessary to make the thickness for the transmissionmode twice the thickness for the reflection mode. The light utilizationfactor is raised by making the thickness for the transmission modelarger than the thickness for the reflection mode.

The refractive indices of birefringent materials forming the λ/4 waveplates 7 and 10 with respect to the ordinary ray and extraordinary raystrongly depend on the wavelength. Therefore, the phase delayaccumulated in the wavelength at a specific thickness also depends onthe wavelength. The phase delay of λ/4 can be completely provided to thelinearly polarized light face of the incident light only when theincident light has a single specified wavelength. Accordingly, in thearea where the phase delay of λ/4 is not achieved due to the wavelengthdependency of the refractive index anisotropy of the birefringentmaterial forming the λ/4 wave plates 7 and 10, a part of the light istransmitted through the polarizer 6 without being absorbed by polarizer6. As a result, the darkness of the black display changes. In the fourthexample, the slower optic axes of the λ/4 wave plates 7 and 10 are setto be perpendicular to each other, and the transmission axes of thepolarizers 6 and 9 are set to be perpendicular to each other. Due tosuch a structure, in the transmission mode, the wavelength dependency ofthe refractive index anisotropy of the λ/4 wave plate 10 is counteractedby the wavelength dependency of the refractive index anisotropy of theλ/4 wave plate 7. Thus, the λ/4 condition is fulfilled in the entirerange from 400 nm to 700 nm. Thus, the darkness of the black display isimproved.

When another phase compensation element is provided at least one ofbetween the polarizer 6 and the liquid crystal layer 5 and between thepolarizer 9 and the liquid crystal layer 5 to improve the viewing angle,satisfactory display is realized in a wide viewing angle.

In the fourth example, the liquid crystal layer 5 is vertically aligned.In the case where the liquid crystal molecules in the vicinity of thesubstrates have a certain tilt angle with respect to the verticaldirection to the substrates, the retardation is not completely zero evenwhen no voltage is applied. By providing a phose compensation element inlieu of the λ/4 wave plate 7 for compensating for the retardation,better black display is obtained.

In the case where the liquid crystal layer has a remaining retardationof a in the reflection mode, a phase compensation element having aretardation of λ/4-α can be provided in lieu of the λ/4 wave plate 7.

In the reflection mode, elliptically polarized light which is offsetfrom the circularly polarized light by the remaining retardation of theliquid crystal layer is incident on the liquid crystal layer. Theelliptically polarized light becomes circularly polarized light whenreaching the reflective electrode after being transmitted through theliquid crystal layer. As a result of the reflection, the light becomescircularly polarized light having an opposite rotation direction. Thelight becomes elliptically polarized light offset from the circularlypolarized light when being transmitted through and going out of theliquid crystal layer. The elliptically polarized light at this point hasthe phase at the time of incidence offset at 90 degrees. When beingtransmitted through the phase compensation element, the ellipticallypolarized light becomes linearly polarized light perpendicular to thetransmission axis of the polarizer 6.

In the case where mainly reflection-mode display is performed, such as,for example, when the reflective pixel electrodes are larger than thetransmissive pixel electrodes, the λ/4 wave plate 10 used for display inthe transmission mode can stay as it is.

As can be appreciated, even when the retardation remaining in thevertically aligned liquid crystal layer is not negligible, high contrastdisplay is obtained in the reflection mode by providing a phasecompensation element in consideration of the retardation.

In the case where the liquid crystal layer has a remaining retardationof α in the reflection mode and a remaining retardation of β in thetransmission mode, a phase compensation element having a retardation ofλ/4-α can be provided in lieu of the λ/4 wave plate 7 and a phasecompensation element having a retardation of λ/4-(β-α) can be providedin lieu of the λ/4 wave plate 10.

In the transmission mode using the light transmitted through a regionhaving a transmission function such as the transmissive electroderegion, when the liquid crystal molecules are aligned vertically to thesubstrates, a phase compensation element having a retardation ofλ/4-(β-α) is set so that the light going out of the liquid crystal layeris elliptically polarized light in the same state as in the reflectionmode. The elliptically polarized light having such a phase difference isincident on the phase compensation element having a retardation ofλ/4-α. Thus, when being transmitted through the phase compensationelement having a retardation of λ/4-α, the light becomes linearlypolarized light perpendicular to the transmission axis of the polarizer6. Accordingly, black display with very little optical leakage isobtained.

As can be appreciated, even when the retardation remaining in thevertically aligned liquid crystal layer is not negligible, high contrastdisplay is obtained in the reflection mode by providing a phasecompensation element in consideration of the retardation.

The LCD device in the fourth example uses a vertically aligned liquidcrystal layer, but display is realized by the same principle using ahorizontally aligned liquid crystal layer. In such a case, as a highervoltage is applied, the retardation by the liquid crystal layer isreduced. However, in the state where most of the liquid crystalmolecules except for those in the vicinity of the substrates arevertical to the substrates when a voltage is applied, the liquid crystalmolecules in the vicinity of the substrates hardly move due to theelectric field. Accordingly, remaining retardation occurs due to theseliquid crystal molecules in the vicinity of the substrates. As can beappreciated, when the horizontally aligned liquid crystal layer is used,optical leakage occurs during the black display and the contrast isreduced by the influence of the remaining retardation compared to whenthe vertically aligned liquid crystal layer is used. In order to realizethe same quality of black display with the horizontally aligned liquidcrystal layer as provided by the vertically aligned liquid crystallayer, the liquid crystal molecules in the vicinity of the substratesneed be aligned so as to interact the remaining retardations by theliquid crystal molecules, or a phase compensation element need beadditionally provided.

EXAMPLE 5

An LCD device in a fifth example according to the present invention willbe described with reference to FIG. 17.

A substrate 2 includes a reflective electrode 3 (shown as reflectiveelectrode region in FIG. 17) formed of a material having a highreflectance such as, for example, Al or Ta and a transmissive electrode8 (shown as transmissive electrode region in FIG. 17) formed of amaterial having a high transmittance such as, for example, ITO. Asubstrate 1 includes a counter electrode 4 (shown as transmissiveelectrode in FIG. 17). A liquid crystal layer 5 formed of a liquidcrystal material exhibiting negative dielectric anisotropy is interposedbetween the reflective electrode 3/transmissive electrode 8 and thecounter electrode 4.

Alignment layers (not shown) are provided on surfaces of the reflectiveelectrode 3/transmissive electrode 8 and the counter electrode 4 whichare in contact with the liquid crystal layer 5. The alignment layers areused to align liquid crystal molecules (not shown) in the liquid crystallayer 5 vertically to surfaces of the substrates 1 and 2. After thealignment layers are provided, at least one of the alignment layers isprocessed with alignment treatment such as, for example, rubbing. Thealignment direction can be defined by optical alignment or electrodeshape in lieu of rubbing.

Due to the alignment treatment, the liquid crystal molecules in theliquid crystal layer 5 has a tilt angle of about 0.1 to 5 degrees withrespect to the vertical direction to the surfaces of the substrates 1and 2.

The reflective electrode 3 is used for applying a voltage to the liquidcrystal layer 5, but the reflective electrode 3 can be used only as areflective plate but not as an electrode for applying a voltage. In sucha case, for example, the transmissive electrode 8 can be extended ontothe reflective electrode 3 to act as an electrode for applying a voltageto the liquid crystal layer 5 in the reflective region.

The liquid crystal material used herein has a refractive indexanisotropy of Ne (refractive index with respect to extraordinaryray)=1.5546 and No (refractive index with respect to ordinaryray)=1.4773.

A λ/4 wave plate 7 is provided on the surface of the substrate 1opposite to the counter electrode 4. A λ/4 wave plate 10 is provided onthe surface of the substrate 2 opposite to the reflective electrode 3and the transmissive electrode 8. A slower optic axis of the λ/4 waveplate 10 is set to be perpendicular to the slower optic axis of the λ/4wave plate 7.

A λ/2 wave plate 11 is provided on the surface of λ/4 wave plate 7opposite to the substrate 1. A λ/2 wave plate 12 is provided on thesurface of the λ/4 wave plate 10 opposite to the substrate 2. A sloweroptic axis of the λ/2 wave plate 11 is set to be tilted at 60 degreeswith respect to the λ/4 wave plate 7. A slower optic axis of the λ/2wave plate 12 is set to be perpendicular to the slower optic axis of theλ/2 wave plate 11.

A polarizer 6 is provided on a surface of the λ/2 wave plate 11 oppositeto the substrate 1. A polarizer 9 is provided on a surface of the λ/2wave plate 12 opposite to the substrate 2. A transmission axis of thepolarizers 6 is set to be tilted at 75 degrees with respect to theslower optic axis of the λ/4 wave plate 7 in such a direction as tointerpose the slower optic axis of the λ/2 wave plate 11, and is set tobe tilted at 15 degrees with respect to the slower optic axis of the λ/2wave plate 11. A transmission axis of the polarizer 9 is set to betilted at 75 degrees with respect to the slower optic axis of the λ/4wave plate 10 in such a direction as to interpose the slower optic axisof the λ/2 wave plate 12, and is set to be tilted at 15 degrees withrespect to the slower optic axis of the λ/2 wave plate 12. Thetransmission axis of the polarizer 6 is set to be perpendicular to thetransmission axis of the polarizer 9.

FIG. 8A is a plan view of an active matrix substrate (substrate 2) inthe second example, and FIG. 8B is a cross-sectional view of the activematrix substrate taken along line 8B-8B′ of FIG. 8A.

As shown in FIGS. 8A and 8B, the active matrix substrate includes a gateline 21, a data line 22, a driving element 23, a drain electrode 24, astorage capacitance electrode 25, a gate insulating layer 26, aninsulating substrate 27, a contact hole 28, an interlayer insulatinglayer 29, a reflective pixel electrode (reflective electrode region) 30(corresponding to the reflective electrode 3 in FIG. 17), and atransmissive pixel electrode (transmissive electrode region) 31(corresponding to the transmissive electrode 8 in FIG. 17).

The storage capacitance electrode 25 is electrically connected to thedrain electrode 24, and overlaps the gate line 21 with the gateinsulating layer 26 being interposed therebetween. Thus, the storagecapacitance electrode 25, the insulating layer 26 and the gate line 21form a storage capacitance.

The contact hole 28 is formed in the interlayer insulating layer 29 forconnecting the transmissive pixel electrode 31 and the storagecapacitance electrode 25.

The active matrix substrate includes a reflective pixel electrode 30 forreflecting the external light and a transmissive pixel electrode 31 forallowing the light from the backlight to be transmitted therethrough inone pixel area.

In FIG. 8B, the reflective electrode 30 has a flat surface, but can havea wave-like surface in order to improve the reflectance. One pixelelectrode includes the reflective pixel electrode 30 and thetransmissive pixel electrode 31 in this example. Alternatively, asemi-transmissive and semi-reflective electrode is usable.

With reference to FIGS. 18A, 18B, 18C and 18D, light transmittance andreflectance in the transmission mode and the reflectance mode of the LCDdevice in the fifth example will be described.

FIGS. 18A and 18B show the reflection mode using the reflectiveelectrode. FIG. 18A shows the black display when no voltage is appliedacross the vertically aligned liquid crystal layer, and FIG. 18B showsthe white display when a voltage is applied across the verticallyaligned liquid crystal layer. FIGS. 18C and 18D show the transmissionmode using the transmissive electrode. FIG. 18C shows the black displaywhen no voltage is applied across the vertically aligned liquid crystallayer, and FIG. 18D shows the white display when a voltage is appliedacross the vertically aligned liquid crystal layer.

With reference to FIG. 18A, black display in the reflection mode will bedescribed.

The light incident on the polarizer 6 is transmitted through thepolarizer 6 to be linearly polarized light parallel to the transmissionaxis of the polarizer 6 and then is incident on the λ/2 wave plate 11.

The λ/2 wave plate 11 is arranged so that the transmission axis of thepolarizer 6 and the slower optic axis of the λ/2 wave plate 11 make anangle of 15 degrees. Thus, the light transmitted through the λ/2 waveplate 11 becomes linearly polarized light tilted at 30 degrees withrespect to the transmission axis of the polarizer 6 in such a directionas to interpose the slower optic axis of the λ/2 wave plate 11. Then,the light is incident on the λ/4 wave plate 7.

The λ/4 wave plate 7 is arranged so that the slower optic axis thereofis tilted at 75 degrees with respect to the transmission axis of thepolarizer 6 in such a direction as to interpose the slower optic axis ofthe λ/2 wave plate 11. In other words, the slower optic axis of the λ/4wave plate 7 is set to be 45 degrees with respect to the polarizationdirection of the linearly polarized light from the λ/2 wave plate 11,and thus the light transmitted through the λ/4 wave plate 7 iscircularly polarized light.

When no voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules exhibiting negative dielectric anisotropy used in theliquid crystal layer 5 are substantially vertical to the surfaces of thesubstrates 1 and 2. Accordingly, the refractive index anisotropy of theliquid crystal layer 5 to the incident light is very small. In otherwords, the phase difference caused by the transmission of the lightthrough the liquid crystal layer 5 is substantially zero.

Accordingly, the circularly polarized light from the λ/4 wave plate 7 istransmitted through the liquid crystal layer 5 as almost beingcircularly polarized and reflected by the reflective electrode 3 in thesubstrate 2.

The circularly polarized light reflected by the reflective electrode 3becomes circularly polarized light having an opposite rotationdirection, and is transmitted through the λ/4 wave plate 7 to belinearly polarized light perpendicular to the light which was incidenton the λ/4 wave plate 7 from the polarizer 6. Then, the light isincident on the λ/2 wave plate 11.

The linearly polarized light from the λ/2 wave plate 11 is perpendicularto the transmission axis of the polarizer 6. Such light is absorbed bythe polarizer 6 without being transmitted.

In this manner, black display is performed.

With reference to FIG. 18B, white display in the reflection mode will bedescribed.

The process until the light is transmitted through λ/4 wave plate 7 tobe circularly polarized light is the same as above and will not bedescribed.

When a voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules are slightly tilted toward the horizontal directionwith respect to the surfaces of the substrates 1 and 2. Accordingly, thecircularly polarized light incident on the liquid crystal layer 5 fromthe λ/4 wave plate 7 becomes elliptically polarized light by thebirefringence of the liquid crystal molecules. The light is thenreflected by the reflective electrode 3, and further influenced by thebirefringence of the liquid crystal molecules in the liquid crystallayer 5. After being transmitted through the λ/4 wave plate 7 and theλ/2 wave plate 11, the light does not become linearly polarized lightperpendicular to the transmission axis of the polarizer 6. Thus, thelight is transmitted through the polarizer 6.

By controlling the voltage applied across the liquid crystal layer 5,the amount of light transmitted through the polarizer 6 after beingreflected by the reflective electrode 3 can be adjusted. Thus, grayscale display is provided.

When a voltage is applied across the liquid crystal layer 5 by thereflective electrode 3 and the counter electrode 4 to change thealignment of the liquid crystal molecules so that the phase differenceby the liquid crystal layer 5 fulfills the ¼ wavelength condition, thecircularly polarized light from the λ/4 wave plate 7 becomes linearlypolarized light perpendicular to the transmission axis of the polarizer6 when reaching the reflective electrode 3 after being transmittedthrough the liquid crystal layer 5. The light is again transmittedthrough the liquid crystal layer 5 to be circularly polarized light andthen transmitted through the λ/4 wave plate 7 and the λ/2 wave plate 11to be linearly polarized light parallel to the transmission axis of thepolarizer 6. In this case, the amount of light transmitted through thepolarizer 6 is maximum.

FIG. 18B shows the case having the liquid crystal layer retardationconditions by which a maximum amount of light reflected by thereflective electrode 3 is transmitted through the polarizer 6. In otherwords, the light on the reflective electrode 3 is linearly polarizedlight perpendicular to the transmission axis of the polarizer 6.

As described above, when no voltage is applied across the liquid crystallayer 5, black display is obtained since the liquid crystal layer 5 hassubstantially no birefringence; and when a voltage is applied across theliquid crystal layer 5, gray scale display is obtained by changing thelight transmittance in accordance with the voltage.

With reference to FIG. 18C, black display in the transmission mode willbe described.

The light emitted by a light source (not shown) is incident on thepolarizer 9 to be linearly polarized light parallel to the transmissionaxis of the polarizer 9. Then, the light is incident on the λ/2 waveplate 12.

The λ/2 wave plate 12 is arranged so that the slower optic axis thereofis tilted at 15 degrees with respect to the transmission axis of thepolarizer 9 and further to be perpendicular to the slower optic axis ofthe λ/2 wave plate 11. Thus, the light transmitted through the λ/2 waveplate 12 is linearly polarized light tilted at 30 degrees with respectto the transmission axis of the polarizer 9 in such a direction as tointerpose the slower optic axis of the λ/2 wave plate 12. Then, thelight is incident on the λ/4 wave plate 10.

The λ/4 wave plate 10 is arranged so that the slower optic axis thereofis tilted at 75 degrees with respect to the transmission axis of thepolarizer 9 in such a direction as to interpose the slower optic axis ofthe λ/2 wave plate 12. In other words, the slower optic axis of the λ/4wave plate 10 is set to be 45 degrees with respect to the polarizationdirection of the linearly polarized light from the λ/2 wave plate 12,and thus the light transmitted through the λ/4 wave plate 10 iscircularly polarized light.

When no voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules exhibiting negative dielectric anisotropy used in theliquid crystal layer 5 are substantially vertical to the surfaces of thesubstrates 1 and 2. Accordingly, the refractive index anisotropy of theliquid crystal layer 5 to the incident light is very small. In otherwords, the phase difference caused by the transmission of the lightthrough the liquid crystal layer 5 is substantially zero.

Accordingly, the circularly polarized light from the λ/4 wave plate 10is transmitted through the liquid crystal layer 5 as almost beingcircularly polarized and is incident on the λ/4 wave plate 7.

The slower optic axes of the λ/4 wave plates 10 and 7 are set to beperpendicular to each other. Thus, the circularly polarized light fromthe λ/4 wave plate 7 becomes linearly polarized light perpendicular tothe transmission axis of the polarizer 8 and then is incident on the λ/2wave plate 11.

The linearly polarized light transmitted through the λ/2 wave plate 11is perpendicular to the transmission axis of the polarizer 6. Such lightis absorbed by the polarizer 6 without being transmitted.

In this manner, black display is performed.

With reference to FIG. 18D, white display in the transmission mode willbe described.

The process until the light is transmitted through λ/4 wave plate 10 tobe circularly polarized light is the same as above and will not bedescribed.

When a voltage is applied across the liquid crystal layer 5, the liquidcrystal molecules are slightly tilted toward the horizontal directionwith respect to the surfaces of the substrates 1 and 2. Accordingly, thecircularly polarized light incident on the liquid crystal layer 5 fromthe λ/4 wave plate 10 becomes elliptically polarized light by thebirefringence of the liquid crystal molecules. The light does not becomelinearly polarized light perpendicular to the transmission axis of thepolarizer 6 even after being transmitted through the λ/4 wave plate 7and the λ/2 wave plate 11 and a part of the light is transmitted throughthe polarizer 6.

By controlling the voltage applied across the liquid crystal layer 5,the amount of light transmitted through the polarizer 6 can be adjusted.Thus, gray scale display is provided.

When a voltage is applied across the liquid crystal layer 5 so that thephase difference by the liquid crystal layer 5 fulfills the ½ wavelengthcondition, the circularly polarized light from the λ/4 wave plate 10becomes linearly polarized light perpendicular to the transmission axisof the polarizer 6 at the half of the thickness of the liquid crystallayer 5, and then becomes circularly polarized light when beingcompletely transmitted through the liquid crystal layer 5.

Since the circularly polarized light from the liquid crystal layer 5becomes linearly polarized light parallel to the transmission axis ofthe polarizer 6 when being transmitted through the λ/4 wave plate 7 andthe λ/2 wave plate 11, most of the light incident on the polarizer 6 istransmitted therethrough. In this case, the amount of light transmittedthrough the polarizer 6 is maximum.

FIG. 18D shows the case having the liquid crystal layer retardationconditions by which a maximum amount of light transmitted through thepolarizer 9 is transmitted through the polarizer 6.

As described above, when no voltage is applied across the liquid crystallayer 5, black display is obtained since the liquid crystal layer 5 hassubstantially no birefringence; and when a voltage is applied across theliquid crystal layer 5, gray scale display is obtained by changing thelight transmittance in accordance with the voltage.

The phase difference by the liquid crystal layer 5 at which thereflectance is maximum in the bright display in the reflection mode isλ/4, and the phase difference by the liquid crystal layer 5 at which thereflectance is maximum in the bright display in the transmission mode isλ/2. As is appreciated from this, when the thickness of the liquidcrystal layer in the reflective region and the thickness of the liquidcrystal in the transmissive region are equal to each other, the phasedifferences of λ/4 for the reflection mode and λ/4 for the transmissionmode cannot be fulfilled at the same time.

In the case where display is performed by changing the phase differenceof the liquid crystal layer in the reflective region from 0 to λ/4, asatisfactory light utilization factor cannot be obtained in thetransmission mode since the phase difference of the liquid crystal layerin the transmissive region also changes only from 0 to λ/4.

Satisfactory light utilization factors both in the reflection mode andthe transmission mode are achieved by changing the thickness of theliquid crystal layer in the reflective region from the thickness in thetransmissive region, or by applying different voltages to the liquidcrystal layer in the reflection region and to the liquid crystal layerin the transmissive region. In the case where the thickness of theliquid crystal layer in the transmission region is made twice thethickness of the liquid crystal layer in the reflective region, thephase differences of the liquid crystal layer of λ/4 for the reflectionmode and of λ/2 for the transmission mode are fulfilled at the sametime. It is not necessary to make the thickness for the transmissionmode twice the thickness for the reflection mode. The light utilizationfactor is raised by making the thickness for the transmission modelarger than but not exceeding twice the thickness for the reflectionmode.

In the fifth example, the slower optic axis of the λ/4 wave plate 10 isset to be perpendicular to the slower optic axis of the λ/4 wave plate7, the slower optic axis of the λ/2 wave plate 12 is set to beperpendicular to the slower optic axis of the λ/2 wave plate 11, and thetransmission axis of the polarizer 6 is set to be perpendicular to thetransmission axis of the polarizer 9. The present invention is notlimited to such setting. Black display is realized when linearlypolarized light transmitted through the polarizer 9 with no retardationby the liquid crystal layer 5 is incident on the polarizer 6 as beingperpendicular to the transmission axis of the polarizer 6 in thetransmission mode.

More specifically, as long as the following conditions are fulfilled,the black display is realized with no birefringence in the liquidcrystal layer 5 and the grey scale display is realized by changing thelight transmittance in accordance with the voltage, even without theabove setting: Where the angle made by the transmission axis of thepolarizer 6 and the slower optic axis of the λ/2 wave plate 11 is γ1,the angle made by the transmission axis of the polarizer 6 and theslower optic axis of the λ/4 wave plate 7 is 2γ1+45 degrees; where theangle made by the transmission axis of the polarizer 9 and the sloweroptic axis of the λ/2 wave plate 12 is γ2, the angle made by thetransmission axis of the polarizer 9 and the slower optic axis of theλ/4 wave plate 10 is 2γ2+45 degrees; and the linearly polarized lighttransmitted through the polarizer 9 with no retardation of the liquidcrystal layer 5 is incident on the polarizer 6 as being perpendicular tothe transmission axis of the polarizer 6 in the transmission mode.

The refractive indices of birefringent materials forming the λ/4 waveplates 7 and 10 and the λ/2 wave plate 11 and 12 with respect to theordinary ray and extraordinary ray strongly depend on the wavelength.Therefore, the phase delay accumulated in the wavelength at a specificthickness also depends on the wavelength. The phase delay of λ/4 can becompletely provided to the linearly polarized light face of the incidentlight only when the incident light has a single specified wavelength.Accordingly, in the area where the phase delay of λ/4 is not achieveddue to the wavelength dependency of the refractive index anisotropy ofthe birefringent material forming the λ/4 wave plates 7 and 10 and theλ/2 wave plate 11 and 12, a part of the light is transmitted through thepolarizer 6 without being absorbed by polarizer 6. As a result, thedarkness of the black display changes. In the fifth example, the λ/4wave plate 7 is combined with the λ/2 wave plate 11 and the λ/4 waveplate 10 is combined with the λ/2 wave plate 12. Due to such astructure, the wavelength dependency of the refractive index anisotropyof the λ/4 wave plate 10 is counteracted to some extent by thewavelength dependency of the refractive index anisotropy of the λ/4 waveplate 7. Thus, the λ/4 condition is fulfilled in a relatively widewavelength range. Thus, the darkness of the black display is improved.

Needless to say, the darkness of the black display can be improvedwithout setting the slower optic axis of the λ/4 wave plate 10 to beperpendicular to the slower optic axis of the λ/4 wave plate 7 andsetting the slower optic axis of the λ/2 wave plate 12 to beperpendicular to the slower optic axis of the λ/2 wave plate 11.

In this example, γ1=γ2=15 degrees, but the values of γ1 and γ2 can bechanged in accordance with a desired darkness. The λ/2 wave plate 12 canbe eliminated to improve the cost performance although the darkness inthe black display is deteriorated in the transmission mode. In thiscase, however, the angle of the slower optic axis of the λ/4 wave plate10 and the transmission axis of the polarizer 9 need be set so that thelinearly polarized light transmitted through the polarizer 9 with noretardation in the liquid crystal layer 5 is incident on the polarizer 6as being perpendicular to the transmission axis of the polarizer 6 inthe transmission mode.

In the case where the slower optic axis of the λ/4 wave plate 10 is setto be perpendicular to the slower optic axis of the λ/4 wave plate 7,the slower optic axis of the λ/2 wave plate 12 is set to beperpendicular to the slower optic axis of the λ/2 wave plate 11, and thetransmission axis of the polarizer 6 is perpendicular to thetransmission axis of the polarizer 9, the following effects are obtainedin the transmission mode. The wavelength dependency of the refractiveindex anisotropy of the λ/4 wave plate 10 is counteracted by thewavelength dependency of the refractive index anisotropy of the λ/4 waveplate 7, and the wavelength dependency of the refractive indexanisotropy of the λ/2 wave plate 12 is counteracted by the wavelengthdependency of the refractive index anisotropy of the λ/2 wave plate 11.Thus, the darkness of the black display is further improved.

When another phase compensation element is provided at least one ofbetween the polarizer 6 and the liquid crystal layer 5 and between thepolarizer 9 and the liquid crystal layer 5 to improve the viewing angle,satisfactory display is realized in a wide viewing angle.

In the fifth example, the liquid crystal layer 5 is vertically aligned.In the case where the liquid crystal molecules in the vicinity of thesubstrates have a certain tilt angle with respect to the verticaldirection to the substrates, the retardation is not completely zero evenwhen no voltage is applied. In the reflection mode, when the retardationof α is remaining, a phase compensation element can be provided at leastone of between the polarizer 6 and the liquid crystal layer 5 andbetween the polarizer 9 and the liquid crystal layer 5 in order tocompensate for the retardation and make the retardation closer to zero.Thus, better black display is realized.

In the case where the vertically aligned liquid crystal layer has aremaining retardation of a in the reflection mode, a phase compensationelement having a retardation of λ/4-α can be provided in lieu of the λ/4wave plate 7.

In the reflection mode, elliptically polarized light which is offsetfrom the circularly polarized light by the remaining retardation of theliquid crystal layer is incident on the liquid crystal layer. Theelliptically polarized light becomes circularly polarized light whenreaching the reflective electrode after being transmitted through theliquid crystal layer. As a result of the reflection, the light becomescircularly polarized light having an opposite rotation direction. Thelight becomes elliptically polarized light offset from the circularlypolarized light when being transmitted through and going out of theliquid crystal layer. The elliptically polarized light at this point hasthe phase at the time of incidence offset at 90 degrees. When beingtransmitted through the phase compensation element, the ellipticallypolarized light becomes linearly polarized light perpendicular to thetransmission axis of the polarizer 6.

In the case where mainly reflection-mode display is performed, such as,for example, when the reflective pixel electrodes are larger than thetransmissive pixel electrodes, the λ/4 wave plate 10 used for display inthe transmission mode can stay as it is.

As can be appreciated, even when the retardation remaining in thevertically aligned liquid crystal layer is not negligible, high contrastdisplay is obtained in the reflection mode by providing a phasecompensation element in consideration of the retardation.

In the case where the liquid crystal layer has a remaining retardationof α in the reflection mode and a remaining retardation of β in thetransmission mode, a phase compensation element having a retardation ofλ/4-α can be provided in lieu of the λ/4 wave plate 7 and a phasecompensation element having a retardation of λ/4-(β-α) can be providedin lieu of the λ/4 wave plate 10.

In the transmission mode using the light transmitted through a regionhaving a transmission function such as the transmissive electroderegion, when the liquid crystal molecules are aligned vertically to thesubstrates, a phase compensation element having a retardation ofλ/4-(β-α) is set so that the light going out of the liquid crystal layeris elliptically polarized light in the same state as in the reflectionmode. The elliptically polarized light having such a phase difference isincident on the phase compensation element having a retardation ofλ/4-α. Thus, when being transmitted through the phase compensationelement having a retardation of λ/4-α, the light becomes linearlypolarized light perpendicular to the transmission axis of the polarizer6. Accordingly, black display with very little optical leakage isobtained.

As can be appreciated, even when the retardation remaining in thevertically aligned liquid crystal layer is not negligible, high contrastdisplay is obtained in the reflection mode by providing a phasecompensation element in consideration of the retardation.

The LCD device in the fifth example uses a vertically aligned liquidcrystal layer, but display is realized by the same principle using ahorizontally aligned liquid crystal layer. In such a case, as a highervoltage is applied, the retardation by the liquid crystal layer isreduced. However, in the state where most of the liquid crystalmolecules except for those in the vicinity of the substrates arevertical to the substrates when a voltage is applied, the liquid crystalmolecules in the vicinity of the substrates hardly move due to theelectric field. Accordingly, remaining retardation occurs due to theseliquid crystal molecules in the vicinity of the substrates. As can beappreciated, when the horizontally aligned liquid crystal layer is used,optical leakage occurs during the black display and the contrast isreduced by the influence of the remaining retardation compared to whenthe vertically aligned liquid crystal layer is used. In order to realizethe same quality of black display with the horizontally aligned liquidcrystal layer as provided by the vertically aligned liquid crystallayer, the liquid crystal molecules in the vicinity of the substratesneed be aligned so as to interact the remaining retardations by theliquid crystal molecules, or a phase compensation element need beadditionally provided.

FIG. 19 shows the relationship between the wavelength and thetransmittance of the light in the black display in the transmission modewhen the slower optic axes of the λ/4 wave plates 7 and 10 are parallelto each other and the slower optic axes of the λ/2 wave plates 11 and 12are parallel to each other and when the slower optic axes of the λ/4wave plates 7 and 10 are parallel to each other and no λ/2 wave plate isprovided for comparison.

As shown in FIG. 19, black display with substantially no optical leakageis realized by providing the λ/2 wave plates 11 and 12.

FIG. 20 shows the relationship between the wavelength and thetransmittance of the light in the black display in the transmission modewhen the slower optic axes of the λ/4 wave plates 7 and 10 are parallelto each other and the slower optic axes of the λ/2 wave plates 11 and 12are parallel to each other as in the fifth example and when the sloweroptic axes of the λ/4 wave plates 7 and 10 are perpendicular to eachother and the λ/2 wave plates 11 and 12 are perpendicular to each otherfor comparison.

As shown in FIG. 20, black display with substantially no optical leakageis realized by setting the slower optic axes of the λ/4 wave plates 7and 10 to be perpendicular to each other and also setting the sloweroptic axes of the λ/2 wave plates 11 and 12 to be perpendicular to eachother.

Embodiment 2

As described above, according to the first embodiment of the presentinvention, a transmission- and reflection-type LCD device providingsatisfactory display quality is realized. Among the LCD devices operableboth in the transmission mode and the reflection mode, the LCD deviceusing a semi-transmissive and semi-reflective layer (FIG. 8C) isinferior to the LCD device having a reflective region for performingdisplay in a reflection mode and a transmission region for performingdisplay in a transmission mode (FIG. 8A) in the following point.

When a semi-transmissive and semi-reflective layer formed of metalparticles deposited in a very small thickness is used, the metalparticles need have a relatively large absorption coefficient.Accordingly, the internal absorption of the incident light is large, anda large percentage of incident light is absorbed or scattered and notused for display. Thus, the light utilization factor is relatively low(for example, in one model, 55% of the incident light is not used fordisplay).

When a semi-transmissive and semi-reflective layer having microscopicholes and recesses (comprehensively referred to as “openings”) is used,the structure of the layer is complicated and requires a preciseproduction design. Therefore, it is difficult to control the thicknessof the layer to be uniform. In other words, reproducibility of theelectric characteristics and optical characteristics is notsatisfactory. Thus, it is difficult to control the display quality ofthe LCD device.

In the second embodiment, LCD devices having a transmissive region forperforming display in the transmission mode and a reflective region forperforming display in the reflection mode and characterized in thestructure of the electrodes will be described. In the case where theelectrode structure in the second embodiment and the phase compensationelement in the first embodiment are combined, the display quality isfurther enhanced.

An LCD device having a transmissive region and a reflective regionutilizes ambient light or illuminating light with less loss and has asignificantly higher light utilization factor compared to an LCD deviceusing a half-mirror. A first conductive layer is formed of a transparentconductive material such as, for example, ITO or SnO₂. A secondconductive layer is formed of Al, W, Cr or an alloy thereof. Since boththe first and second conductive layers can be formed of materials usedfor general reflective-type LCD devices and transmission-type LCDdevices, the LCD device provides very stable display characteristics andreliability and is produced relatively easily.

Furthermore, the LCD devices in the second embodiment solve the problemof the conventional transmission-type LCD device that the visibility islowered due to the surface reflection when the ambient light is brightand also the problem of the conventional reflection-type LCD device thatsatisfactory display is not obtained due to the lowered brightness whenthe ambient light is dark. In the circumstances where a sufficient poweris provided, the backlight is utilized as the conventionaltransmission-type LCD devices. Thus, sufficient display is realizedregardless of the ambient light intensity without requiring thedispersion in the utilization factor of the ambient light to becontrolled as precisely as in the conventional reflection-type LCDdevices. When used, the region including the first conductive layerhaving a relatively high transmittance and the region including thesecond conductive layer having a relatively high reflectance which areprovided in one pixel area complementarily contribute to display.Therefore, regardless of the ambient light intensity, clear images aredisplayed.

When adopted in a viewfinder (monitoring screen) of a battery-drivendigital camera or video camera, the LCD device in the second embodimentprovides appropriately bright images, which are easy to observe, byadjusting the brightness of the backlight regardless of the ambientlight intensity.

Especially when used outdoors on a fine day, images provided by theconventional transmission-type LCD devices has a lower contrast evenwhen the brightness of the backlight is raised. The quality of suchimages can be improved while reducing the power consumption by turningoff the backlight and using the LCD device according to the presentinvention in the reflection mode, or by lowering the brightness of thebacklight and using both the transmission mode and the reflection modeof the LCD device according to the present invention. When the LCDdevice is used indoors receiving bright sunlight, the reflection modeand the transmission mode can be switched in accordance with thedirection of the object, or both modes can be used together to provideeasy-to-see display. When the monitoring screen receives the sunlight,the LCD device can be used in the same manner as used outdoors on a fineday. When the object is display from a dark corner of the room, thebacklight can be turned on to use the LCD device in the transmissionmode.

When adopted in a monitoring of an automobile-mounted car navigationdevice, the LCD device in the second embodiment provides appropriatelybright images, which are easy to observe. The conventionalautomobile-mounted monitoring screen uses a backlight having a higherbrightness than the backlight used in personal computers or the like inorder to deal with the external light incident on the screen. However,the conventional automobile-mounted monitoring screen still has theproblem of lowered contrast. In contrast, the backlight having such ahigh brightness is not suitable for display at night or in the tunnels.The LCD device in the second embodiment provides satisfactory displaywhen the ambient light is bright by using the reflection mode togetherwith the transmission light without setting the brightness of thebacklight high. In the darkness also, easy-to-see display is provided bya brightness of only about 50 to 100 cd/m².

In an LCD device in the second embodiment, a pixel electrode includes afirst conductive layer having a relatively high light transmittance anda second conductive layer having a relatively high light reflectance,which are electrically connected to each other. Thus, a transmissiveregion for performing display in the transmission mode and a reflectiveregion for performing display in the reflection mode are both includedin one pixel area.

The first conductive layer and the second conductive layer are providedin separate layers with an insulating layer being interposedtherebetween. The thickness of the liquid crystal layer can be adjustedby changing the thickness of the insulating layer between thetransmissive region (for the first conductive layer) and in thereflective region (for the second conductive layer). In this manner, theoptical characteristics in the two regions can be matched to each other.During the production process, the two layers having different level ofpotentials are provided with the insulating layer being interposedtherebetween. Therefore, electrocorrosion is not caused with thedeveloper used for forming the electrode by patterning or with theresist remover acting as the electrolyte. Accordingly, a highly reliableLCD device is obtained.

When the two layers of the pixel electrode (e.g., lower layer of ITO andupper layer of Al) are sequentially formed with no insulating layerbeing interposed therebetween, the levels of potentials of the ITO layerand the Al layer are significantly different. Furthermore, the thinfilms has many microscopic openings. Accordingly, the developer used forpatterning or as a resist remover tends to act as an electrolyte tocause electrocorrosion. As a result, the ITO layer is eluted, thuscausing pixel defects, line disconnections, and contamination of theliquid crystal layer. Since the insulating layer is provided between thetwo layers according to the present invention, the insulating layer actsas a protective layer to preventing invasion of the liquids, whichcauses electrocorrosion.

Even when the two layers forming the pixel electrode have therelationship tending to cause electrocorrosion, the two layers can beconnected to each other via a third conductive layer for alleviating theproperties of the two layers. Thus, insufficient connection due toelectrocorrosion and other inconveniences which deteriorate thereliability of the LCD device are prevented.

In the case where one of the first, second and third conductive layersis formed of a part of the materials of the gate line or source line,the production process is simplified.

The surface of the insulating layer on which the second conductive layeris to be formed can be formed wave-like. In such a case, the incidentlight is reflected and scattered, resulting in a wider viewing angle.Thus, paper white display is realized without using an additionalscattering plate.

According to a method for producing an LCD device of the presentinvention, complicated production conditions which are required in thecase of a conventional LCD device using a half-mirror are not necessary.General electrode materials and line materials and general productionconditions used for the conventional transmission-type LCD devices andreflection-type devices can be used. In consequence, the production isrelatively easy and reproducibility is satisfactory. Even when the twolayers forming the pixel electrode have the relationship tending tocause electrocorrosion, the two layers can be connected to each othervia an insulating layer or a third conductive layer can be providedtherebetween. As a result, the two layers can be formed without directcontact or contact with liquids which tend to act as an electrolyte.Thus, electrocorrosion is prevented, and therefore the LCD device enjoysa high reliability and is produced at high efficiency.

In the case where the step of removing the insulating layer in a contactarea with the first and second conductive layers and the step ofremoving the insulating layer on a part of the first conductive layerare performed in the same step, the number of the steps is reduced whilemaintaining the reliability of the LCD device.

EXAMPLE 6

An LCD device in a sixth example according to the present invention willbe described.

FIG. 21 is a plan view of an active matrix substrate of the LCD devicein the sixth example corresponding to one pixel area. FIG. 22 is across-sectional view of the LCD device taken along line 22-22′ in FIG.21.

As shown in FIGS. 21 and 22, a plurality of gate lines 53 and aplurality of source lines 59 a are provided so as to perpendicular toeach other on a transparent insulating plate (not shown) formed of aglass or plastic material. A TFT 57 is provided in the vicinity of eachof intersections of the gate lines 53 and the source lines 59 a. A drainelectrode 59 c of the TFT 57 is connected to a reflective electrode 61and a transmissive electrode 58 a which act as a pixel electrodetogether. A portion of the LCD device having the pixel electrode as atop layer thereof has a region T (transmissive region) having arelatively high light transmittance and a region R (reflective region)having a relatively high light reflectance when seen from above the LCDdevice.

Although not shown, an alignment layer for aligning liquid crystalmolecules is provided on the active matrix substrate shown in FIG. 21.

LCD devices in the sixth and the following examples includes theabove-described active matrix substrate and a counter substrateincluding a transmissive electrode and an alignment layer. Whennecessary, a color filter, a phase compensation element or a polarizercan be provided.

The region T is rectangular and located at a center of the pixelelectrode. In cross-section, the region T includes a plurality of layersformed of materials having a high light reflectance and also includesthe transmissive electrode 58 a as the top layer connected to the drainelectrode 59 c of the TFT 57. The region R surrounds the region T andincludes the reflective electrode 61 as the top layer. The reflectiveelectrode 61 is formed of Al or an Al-containing alloy having a highlight reflectance and connected to the drain electrode 59 c of the TFT57. Due to such a structure, the region R can reflect the incidentlight. The reflective electrode 61, which has a wave-like surface,scatters incident light to an appropriate range of directions.

A liquid crystal material used in the LCD device in the sixth example isthe guest-host liquid crystal material ZLI2327 (produced by Merck & Co.,Inc.) containing a black pigment and also containing an optically activesubstance S-811 (produced by Merck & Co., Inc.) at a ratio of 0.5%.

As shown in FIG. 22, the TFT 57 includes a gate insulating layer 54, asemiconductor layer 55, semiconductor contact layers 56 a and 56 b, asource electrode 59 b, the drain electrode 59 c, and a gate electrode 52on which the above-mentioned elements are provided sequentially. Thegate electrode 52 is branched from each of the gate lines 53 (FIG. 21).

The drain electrode 59 c is connected to the transmissive electrode 58 awhich is a part of the pixel electrode in the region T. In the region R,an interlayer insulating layer 60 and the reflective electrode 61 areprovided sequentially on the transmissive electrode 58 a. The reflectiveelectrode 61 is electrically connected to the transmissive electrode 58a through a contact hole 63 formed in the interlayer insulating layer60. The reflective electrode 61 and the transmissive electrode 58 a formthe pixel electrode for applying a voltage to the liquid crystalmaterial. The transmissive electrode 58 a and the reflective electrode61 are not directly connected to each other but are connected via aconductive metal layer 62 formed of Ti.

The transmissive electrode 58 a can be covered with the interlayerinsulating layer 60 when the reflective electrode 61 is formed bypatterning (a method for producing the LCD device will be describedlater in detail). Accordingly, ITO and Al cause electrocorrosion,thereby effectively preventing inconveniences such as linedisconnection. In the case where the interlayer insulating layer 60 isformed on the transmissive electrode 58 a with a relatively smallthickness so as to completely cover the transmissive electrode 58 a,electrocorrosion is prevented from occurring between ITO and Al afterthe LCD device is produced.

In this example, the metal layer 62 is formed of Ti, but the same effectis obtained as long as the metal layer 62 is formed of a conductivematerial other than Al, for example, Cr, Mo, Ta, or W. Alternatively,the reflective electrode 61 is formed of an Al-containing alloycontaining a metal material added thereto having a higher potential thanAl, for example, W, Ni, Pd, V or Zr, in lieu of forming the metal layer62. In this case also, the electrocorrosion between ITO and Al after theproduction of the LCD device is prevented. For example, theelectrocorrosion is more effectively prevented by adding about 5.0 wt. %of W to Al.

A method for producing the LCD device in this example will be describedwith reference to FIGS. 23A through 23E.

As shown in FIG. 23A, a conductive thin film is formed on the insulatingplate 51 and patterned into a prescribed shape by photolithography,thereby forming the gate electrode 52 and the gate line (not shown). Inthis example, the insulating plate 51 is formed of glass, and the gateelectrode 52 and the gate line are formed of Ta. The insulating plate 51can be formed of plastics or the like in lieu of glass, and the gateelectrode 52 and the gate line can be formed of other conductivematerials such as, for example, Al, Cr, Mo, W, Cu, or Ti.

Next, as shown in FIG. 23B, the gate insulating layer 54 of SiN_(x), thesemiconductor layer 55 of a-Si, and a P-doped n⁺-a-Si layer for thesemiconductor contact layers 56 a and 56 b are sequentially formed byCVD, and then patterned by photolithography.

Then, a conductive layer is formed and patterned by photolithographyinto a prescribed shape, thereby forming the source line 59 a, thesource electrode 59 b and the drain electrode 59 c. The conductive layeris formed of a Cr-containing material in this example, but can be formedof other conductive materials such as, for example, Al, Mo, Ta, W, Cu orTi.

Then, as shown in FIG. 23C, a light-transmissive conductive layer isformed and patterned by photolithography, thereby forming thetransmissive electrode 58 a. The transmissive electrode 58 a is formedof ITO in this example.

Then, a metal layer is formed and patterned by photolithography, therebyforming the metal layer 62. The metal layer 62 is used for connectingthe transmissive electrode 58 a and the reflective electrode 61 to beformed later. The metal layer 62 is formed of Ti in this example, butcan be formed of other conductive materials other than Ti such as, forexample, Cr, Mo, Ta or W.

Then, the P-doped n⁺-a-Si layer is etched using the source electrode 59b and the drain electrode 59 c as masks, thereby forming thesemiconductor contact layers 56 a and 56 b. In this manner, the TFT 57is completed.

The source electrode 59 b and the drain electrode 59 c can overlap thetransmissive electrode 58 a.

Next, as shown in FIG. 23D, the interlayer insulating layer 60 isformed. The interlayer insulating layer 60 is patterned byphotolithography to form the contact hole 63 and to remove a part of theinterlayer insulating layer 60 in the region T. Simultaneously, asurface of the interlayer insulating layer 60 in the region R (on whichthe reflective electrode 61 is to be formed) is formed wave-like.

By removing the part of the interlayer insulating layer 60 in the regionT, the transmittance of the region T can be improved. However, theinterlayer insulating layer 60 is not completely removed but remains ata certain thickness. This prevents electrocorrosion when the reflectiveelectrode 61 is formed by patterning. In other words, the orientationstate of the liquid crystal molecules can be substantially the samewithin each pixel area.

The surface of the interlayer insulating layer 60 in the region Rappears to have a plurality of round protrusions 64 when seen fromabove. In cross-section, the surface area of the interlayer insulatinglayer 60 slowly fluctuates. When the reflective electrode 61 is formedon such a wave-like surface, the incident light is reflected by thewave-like surface of the reflective electrode 61 efficiently andscattered in an appropriate range of directions. The shape of thewave-like surface can be optimally determined in accordance with desireddisplay characteristics. In the case where it is not necessary toscatter the light, the surface need not be wave-like.

The interlayer insulating layer 60 is formed of a single layer(thickness: 2.5 μm) of an organic resin in this example, but can beformed of a plurality of laminated layers of different materials. Arelatively thick single layer of an organic-resin as in this example isadvantageous in that generation of a parasitic capacitance is avoidedeven when the reflective electrode 61 partially overlaps the TFT 57. Asa result, the display quality is improved and the numerical aperture israised. A relatively thick organic resin layer also facilitatesformation of the wave-like surface.

Alternatively, the interlayer insulating layer 60 can be formed of ageneral inorganic layer such as, for example, SiN_(x). Such a layer isadvantageous in obtaining a high insulation although being relativelythin, but is disadvantageous in that formation of the wave-like surfaceis difficult. When it is not necessary to form the wave-like surface dueto the desired display characteristics, such a layer is preferable.

As shown in FIG. 23E, an Al layer is formed and patterned, therebyforming the reflective electrode 61 in the region R. The reflectiveelectrode 61 is electrically connected to the transmissive electrode 58a and the drain electrode 59 c of the TFT 57 through the contact hole 63and the metal layer 62. The reflective electrode 61 is formed of Al inthis example, but can be formed of other Al-containing alloys orconductive materials having a high light reflectance.

Thus, the active matrix substrate shown in FIGS. 21 and 22 is completed.

Although not shown, an alignment layer is formed on top of the activematrix substrate. The active matrix substrate equipped with thealignment layer is combined with a counter substrate including atransmissive electrode and equipped with an alignment layer. A liquidcrystal material is injected into the gap between the two substrates.Thus, the LCD device in the sixth example is completed. When necessary,a color filter or a phase compensation element can be added.

As described above, a liquid crystal material used in the LCD device inthis example is the guest-host liquid crystal material ZLI2327 (producedby Merck & Co., Inc.) containing a black pigment and also containing anoptically active substance S-811 (produced by Merck & Co., Inc.) at aratio of 0.5%.

In this example, the satisfactory display characteristics is obtained bysetting the area ratio to the region T:region T=40:60. The area ratio isnot limited to this but can be appropriately changed in accordance withthe transmittance and reflectance of the regions T and R and the use ofthe LCD device. In this example, only one region T is provided at thecenter of the pixel area. A plurality of regions T can be provided, andthe region T can be of any other shape.

In the LCD devices in the second embodiment, a pixel electrode includesa region T having a relatively high light transmittance located at acenter thereof and a region R having a relatively high light reflectancearranged around the region T. Due to such a structure, the LCD deviceutilizes ambient light and illuminating light with less loss compared tothe conventional LCD devices using a half-mirror. Moreover, the LCDdevices in the second embodiment solve the problem of the conventionaltransmission-type LCD device that the visibility is lowered due to thesurface reflection when the ambient light is bright and also the problemof the conventional reflection-type LCD device that satisfactory displayis not obtained due to the lowered brightness when the ambient light isdark. In other words, the LCD devices in the second embodiment providesatisfactory display regardless of the ambient light intensity withoutrequiring the dispersion in the light utilization factor due to thedispersion in the reflection characteristics to be controlled asprecisely as in the case of the conventional reflection-type LCDdevices.

The LCD device in this example can be produced with the generalelectrode and line materials and conditions used in the conventionaltransmission-type LCD devices and reflection-type LCD devices. Thecomplicated conditions required for the conventional LCD devices using ahalf-mirror are not necessary. Accordingly, the LCD device in thisexample is produced relatively easily with satisfactory reproducibility.Moreover, the display characteristics, which are difficult with theconventional LCD devices using a half-mirror, can be performedrelatively easily.

EXAMPLE 7

An LCD device in a seventh example according to the present inventionwill be described.

FIG. 24 is a plan view of an active matrix substrate of the LCD devicein the seventh example corresponding to one pixel area. FIG. 25 is across-sectional view of the LCD device taken along line 25-25′ in FIG.24.

The LCD device in the seventh example is different from the LCD devicein the sixth example in the structure regarding the electric connectionbetween the reflective electrode 61 as a part of the pixel electrode andthe TFT 57 and the production method regarding the structure.

As shown in FIGS. 24 and 25, the drain electrode 59 c of the TFT 57 isconnected to the transmissive electrode 58 a. The transmissive electrode58 a acts as a part of the pixel electrode for applying a voltage to theliquid crystal material in the region T. In the region R, the interlayerinsulating layer 60 and the reflective electrode 61 are provided on thetransmissive electrode 58 a. The reflective electrode 61 is directlyconnected to the drain electrode 59 c through the contact hole 63. Thereflective electrode 61 also acts as a part of the pixel electrode. Asin the sixth example, the transmissive electrode 58 a formed of ITO isnot directly connected to the reflective electrode 61 formed of Al. Dueto such a structure, the TFT 57 is electrically connected to thesematerials with certainty without any undesirable possibility ofelectrocorrosion or the like, while utilizing the high reflectance ofthe ambient light in the region R and the high transmittance of thelight from the backlight in the region T.

In this example, the electrocorrosion of ITO and Al is prevented. Thepresent invention is effectively applicable to any combination ofdifferent materials which tend to cause electrocorrosion.

Hereinafter, the active matrix substrate shown in FIGS. 24 and 25 willbe described.

The process until the semiconductor layer 55 and a layer to be thesemiconductor contact layers 56 a and 56 b are formed is performed inthe same manner as in the sixth example.

Then, a conductive layer is formed and patterned by photolithography,thereby forming the source line 59 a, the source electrode 59 b, thedrain electrode 59 c and a connecting metal layer 59 d. The conductivelayer is formed of a Cr-containing material in this example, but can beformed of other conductive materials such as, for example, Al, Mo, Ta,W, Cu or Ti.

Then, the transmissive electrode 58 a is formed so as to partiallyoverlap the connecting metal layer 59 d. Alternatively, the connectingmetal layer 59 d can partially overlap the transmissive electrode 58 a.In this example also, the transmissive electrode 58 a is formed of ITO.

The source line 59 a, the source electrode 59 b, the drain electrode 59c and the connecting metal layer 59 d can be formed on the transmissiveelectrode 58 a.

Then, the interlayer insulating layer 60 is formed in the same manner asin the sixth example, and is patterned by photolithography to form thecontact hole 63 and to remove a part of the interlayer insulating layer60 in the region T. Then, the reflective electrode 61 is formed. In thisexample also, the reflective electrode 61 is formed of Al.

As can be appreciated, in this example, the transmissive electrode 58 aformed of ITO is not directly connected to the reflective electrode 61formed of Al. Due to such a structure, generation of malfunction due toelectrocorrosion between ITO and Al is prevented at the contact portion,thus to improve the reliability. The metal layer 62, which is formed ofthe same material of the source electrode 59 b, can be formed relativelyeasily.

EXAMPLE 8

In an eighth example, another methods for producing the LCD devicedescribed in the seventh example will be described with reference toFIG. 26A through 26C.

FIGS. 26A through 26C, corresponding to FIG. 25, are cross-sectionalviews illustrating a method for producing the LCD device described inthe seventh example.

The process until the formation of the interlayer insulating layer 60 isperformed as described in the seventh example.

Then, as shown in FIG. 26A, a part of the interlayer insulating layer 60is removed by photolithography, thereby forming the contact hole 63. Inthe same step, the surface of the interlayer insulating layer 60 in theregion R is formed wave-like so that the incident light is scattered bythe surface. Unlike the seventh example, a part of the interlayerinsulating layer 60 in the region T is not removed.

On the surface of the interlayer insulating layer 60, an Al layer or anAl-containing alloy layer is formed. The interlayer insulating layer 60is formed of a single organic resin layer in this example, but can beformed of a plurality of layers of different materials. The surface ofthe interlayer insulating layer 60 need not be wave-like.

Then, as shown in FIG. 26B, the Al layer is patterned byphotolithography, thereby forming the reflective electrode 61.

Next, as shown in FIG. 26C, the interlayer insulating layer 60 isremoved in a part or the entirety of the region T.

In this manner, the active matrix substrate shown in FIGS. 24 and 25 iscompleted.

An LCD device having such an active matrix substrate is operable both inthe transmission mode and the reflection mode simultaneously. Thereflective electrode 61 formed of Al and the transmissive electrode 58 aformed of ITO are not directly connected to each other after the LCDdevice is completed and thus do not cause electrocorrosion. Therefore,malfunction by electrocorrosion is prevented, thus improving thereliability of the LCD device. During the production process also,electrocorrosion is prevented since the transmissive electrode 58 a isnot exposed to an etchant while the reflective electrode 61 is formed bypatterning.

EXAMPLE 9

An LCD device in a ninth example according to the present invention willbe described.

The LCD device in the ninth example is different from the LCD device inthe seventh and eighth examples in the order of forming the transmissiveelectrode 58 a and the drain electrode 59 c and in the step of formingthe contact hole 63.

FIGS. 27A through 27C, corresponding to FIG. 25, are cross-sectionalviews illustrating a method for producing the LCD device in the ninthexample.

The process until the formation of a layer to be the semiconductorcontact layers 56 a and 56 b is performed as described in the sixth andseventh examples.

As shown in FIG. 27A, a light-transmissive conductive layer is formedand patterned by photolithography, thereby forming the transmissiveelectrode 58 a. In this example, the transmissive electrode 58 a isformed of ITO.

Then, a conductive layer is formed and patterned by photolithography,thereby forming the source line 59 a, the source electrode 59 b, thedrain electrode 59 c, the connecting metal layer 59 d, and a metal layer59 e for the region T. The source electrode 59 b is branched from thesource line 59 a. The drain electrode 59 c, the connecting metal layer59 d and the metal layer 59 e for the region T are electricallyconnected to one another. The conductive layer is formed of aTa-containing material in this example, but can be formed of otherconductive materials such as, for example, Al, Cr, Mo, W, Cu or Ti.

Then, etching is performed using the source electrode 59 b and the drainelectrode 59 c as masks, thereby forming the semiconductor conductlayers 56 a and 56 b. Thus, the TFT 57 is completed.

Next, the interlayer insulating layer 60 is formed. The contact hole 63is formed and also a part of the interlayer insulating layer 60 in theregion T is removed by photolithography. In the same step, the surfaceof the interlayer insulating layer 60 in the region R is formedwave-like so as to scatter the incident light. On the surface, an Allayer or an Al-containing alloy layer is formed. The interlayerinsulating layer 60 is formed of a single organic resin layer in thisexample, but can be formed of a plurality of layers of differentmaterials. The surface of the interlayer insulating layer 60 need not bewave-like.

As shown in FIG. 27B, the Al or Al-containing alloy layer is formed byphotolithography, thereby forming the reflective electrode 61.

Then, as shown in FIG. 27C, the layer 59 e in the region T is partiallyor entirely removed by photolithography or by etching using thereflective electrode 61 as a mask. Alternatively, the layer 59 e can beetched while the reflective electrode 61 is formed by patterned byetching.

As described above, in this example, the reflective electrode 61 formedof Al is not directly connected to the transmissive electrode 58 aformed of ITO. Accordingly, electrocorrosion between Al and ITO isprevented after the LCD device is completed, and thus malfunction byelectrocorrosion is also prevented, thus to improve the reliability.During the production process also, electrocorrosion is prevented sincethe transmissive electrode 58 a is not exposed to an etchant while thereflective electrode 61 is formed by patterning.

EXAMPLE 10

An LCD device in a tenth example according to the present invention willbe described.

The LCD device in the tenth example is different from the LCD device inthe seventh and eighth examples in that the structure of thetransmissive electrode 58 a and the TFT 57 and in the step forming thecontact hole 63.

FIGS. 28A through 28C, corresponding to FIG. 25, are cross-sectionalviews illustrating a method for producing the LCD device in the tenthexample.

The process until the formation of a layer to be the semiconductorcontact layers 56 a and 56 b is performed as described in the sixth andseventh examples.

As shown in FIG. 28A, a light-transmissive conductive layer and a metallayer are sequentially formed. The metal layer is patterned byphotolithography, thereby forming an upper half of the source line 59 a,an upper half of the source electrode 59 b, an upper half of the drainelectrode 59 c, the connecting metal layer 59 d, and the metal layer 59e for the region T. Then, the light-transmissive conductive layer ispatterned in the same patterns as those of the source line 59 a, anupper half of the source electrode 59 b, an upper half of the drainelectrode 59 c, the connecting metal layer 59 d, and the metal layer 59e. Thus, a lower half of the source line, a lower half of the sourceelectrode 58 b, a lower half of the drain electrode 58 c, and thetransmissive electrode 58 a.

As can be appreciated from the above, the source line, the sourceelectrode and the drain electrode have a two-layer structure. Even whena disconnection or any other malfunction occurs in one of the twolayers, a normal signal is sent through the other layer, thus realizingnormal display.

In this example, the light-transmissive conductive layer is formed ofITO and the metal layer is formed of a Ta-containing material. Thelight-transmissive conductive layer can be etched successively from themetal layer or can be etched, after the mask for the metal layer isremoved, using a separate mask.

Next, etching is performed using the source electrode 59 b/58 b and thedrain electrode 59 c/58 c as masks, thereby forming the semiconductorcontact layers 56 a and 56 b. Thus, the TFT 57 is completed.

Then, the interlayer insulating layer 60 is formed. The contact hole 63is formed and also a part of the interlayer insulating layer 60 in theregion T is removed by photolithography. In the same step, the surfaceof the interlayer insulating layer 60 in the region R is formedwave-like so as to scatter the incident light. On the surface, an Allayer or an Al-containing alloy layer is formed. The interlayerinsulating layer 60 is formed of a single organic resin layer in thisexample, but can be formed of a plurality of layers of differentmaterials. The surface of the interlayer insulating layer 60 need not bewave-like.

As shown in FIG. 28B, the Al or Al-containing alloy layer is formed byphotolithography, thereby forming the reflective electrode 61.

Then, as shown in FIG. 28C, the layer 59 e in the region T is partiallyor entirely removed by photolithography or by etching using thereflective electrode 61 as a mask. Alternatively, the layer 59 e can beetched while the reflective electrode 61 is formed by patterned byetching.

As described above, in this example, the reflective electrode 61 formedof Al is not directly connected to the transmissive electrode 58 aformed of ITO. Accordingly, electrocorrosion between Al and ITO isprevented after the LCD device is completed, and thus malfunction byelectrocorrosion is also prevented, thus to improve the reliability.During the production process also, electrocorrosion is prevented sincethe transmissive electrode 58 a is not exposed to an etchant while thereflective electrode 61 is formed by patterning. Furthermore, since thepixel electrode (transmissive electrode 58 a) is formed in the same stepas the other lines and electrodes, the production method is simplified.

The pixel electrode (transmissive electrode 58 a) is formed in the samestep as the source line, the source electrode and the drain electrode inthis example, but can be formed in the same step as the gate line andthe gate electrode. In lieu of the transmissive electrode 58 a, thereflective electrode can be formed in the same step as the other linesand electrodes.

EXAMPLE 11

An LCD device in an eleventh example according to the present inventionwill be described. Also described is a structure of a terminal sectionand a method for forming the same.

The LCD device in the eleventh example is different from the LCD devicein the sixth through tenth examples in that the transmissive electrode58 a is provided in the same layer as the gate electrode and the gateline.

FIGS. 29A through 29C are cross-sectional views illustrating a methodfor producing the LCD device in the eleventh example, especially theactive matrix substrate and the terminal section of the LCD device. FIG.29A, corresponding to FIG. 25, shows a structure of a display section ofthe LCD device. FIG. 30 is a plan view of the LCD device in the eleventhexample. FIG. 29B, which is a cross-sectional view of the LCD devicetaken along line 29B-29B′ in FIG. 30, shows a terminal structure of agate terminal section. FIG. 29C, which is a cross-sectional view of theLCD device taken along line 29C-29C′ in FIG. 30, shows a terminalstructure of a source terminal section.

As shown in FIG. 29A, the TFT 57 is provided on the insulating plate 51.The transmissive electrode 58 a is provided in the same layer as thegate electrode 52 of the TFT 57 and the gate line (not shown). The drainelectrode 59 c is connected to the reflective electrode 61 through thecontact hole 63 in the interlayer insulating layer 60 and also connectedto the transmissive electrode 58 a through a contact hole 63 formed inthe gate insulating layer 54.

According to such a structure, after the transmissive electrode 58 a isformed but before the reflective electrode 61 is completely formed inthe same pixel area as the transmissive electrode 58 a, at least thetransmissive electrode 58 a is covered with the gate insulating layer54. Therefore, generation of electrocorrosion due to the potentialdifference between the electrode 58 a and 61 is prevented.

In the gate and source terminal sections shown in FIGS. 29B and 29Calso, a gate line (70 and 53) and a source line 71 which are formed inthe same layer as the transmissive electrode 58 a are covered with thegate insulating layer 54 and the interlayer insulating layer 60.Accordingly, the gate line (70 and 53) and the source line 71 arecovered with the insulating layers until the reflective electrode 61 iscompletely formed on the interlayer insulating layer 61. Thus,electrocorrosion between the gate line 70+53/source line 71 and thereflective electrode 61 formed of different metal materials isprevented.

With reference to FIGS. 31A through 31E and 32A through 32C, a methodfor producing the LCD device in the eleventh example will be describedregarding the display section.

As shown in FIG. 31A, a light-transmissive conductive layer is formed onthe insulating plate 51 and patterned by photolithography, therebyforming the transmissive electrode 58 a. In this example, the insulatingplate 51 is formed of glass, and the transmissive electrode 58 a isformed of ITO.

Then, the gate electrode 52 and the gate line (not shown) are formed byforming a layer on the insulating plate 51 and patterning the layer byphotolithography. The gate electrode 52 and the gate line are formed ofa Ta-containing material in this example, but can be formed of otherconductive materials such as, for example, Al, Cr, Mo, W, Cu or Ti.

The gate electrode 52 and the gate line can be formed before thetransmissive electrode 58 a.

Then, as shown in FIG. 31B, the gate insulating layer 54 of SiN_(x), thesemiconductor layer 55 of a-So, and a P-doped n⁺-a-So layer for thesemiconductor contact layers 56 a and 56 b are sequentially formed byCVD, and then patterned by photolithography.

The contact hole 63 is formed in the gate insulating layer 54 forelectrically connecting the transmissive electrode 58 a and the drainelectrode 59 c to be formed later to each other.

The gate insulating layer 54 on the gate terminal (FIG. 29B) and thesource terminal (FIG. 29C) in the gate and source terminal sections canbe removed in the same step.

Next, as shown in FIG. 31C, a conductive layer is formed and patternedby photolithography, thereby forming the source line 59 a, the sourceelectrode 59 b and the drain electrode 59 c. The conductive layer isformed of a Cr-containing material in this example, but can be formed ofother conductive materials such as, for example, Al, Mo, Ta, W, Cu orTi.

Then, etching is performed using the source electrode 59 b and the drainelectrode 59 c, thereby forming the semiconductor contact layers 56 aand 56 b. Thus, the TFT 57 is completed.

As shown in FIG. 31D, the interlayer insulating layer 60 is formed, andthe contact hole 63 is formed in the interlayer insulating layer 60 byphotolithography. A part of the interlayer insulating layer 60 in theregion T is not removed in this step, but after the reflective electrode61 is formed.

As shown in FIG. 31E, the surface of the interlayer insulating layer 60is formed wave-like by photolithography.

The interlayer insulating layer 60 is formed of a single layer of anorganic insulating material in this example, but can be formed of aplurality of layers of different materials. The surface of theinterlayer insulating layer 60 need not be wave-like.

Then, as shown in FIG. 32A, a conductive layer having a relatively highreflectance is formed on the surface of the interlayer insulating layer60.

As shown in FIG. 32B, the conductive layer is patterned byphotolithography, thereby forming the reflective electrode 61. Thereflective electrode 61 is not formed at least in the region T.

Then, as shown in FIG. 32C, the part of the interlayer insulating layer60 in the region T is removed. A part of the gate insulating layer 54 inthe region T is also removed. Both the insulating layers 54 and 60 arepreferably removed from the region T since the layers may undesirablycause a voltage drop, thus preventing sufficient voltage to the liquidcrystal material. Especially in the case where a voltage is appliedacross the liquid crystal material by the transmissive electrode 58 aand the reflective electrode 61 which are electrically connected to eachother, the existence of the insulating layers in the region T causes adifference between the voltages applied across the liquid crystalmaterial in the region T and the region R and thus is not preferable.

In this manner, the active matrix substrate shown in FIG. 29A iscompleted.

An alignment layer is formed on the active matrix substrate, andalignment treatment is performed to the alignment layer when necessary.Then, the active matrix substrate is combined with a counter electrode.A liquid crystal material is injected into the gap between thesubstrates. Thus, the LCD device in the eleventh example is completed.

With reference to FIGS. 33A through 33F, a method for forming the gateterminal section will be described. The gate terminal section can beformed in the same steps as those of the display section.

As shown in FIG. 33A, a light-transmissive conductive layer acting as alower layer 70 of the gate line is formed on the insulating plate 51. Inthe same step, the transmissive electrode 58 a (FIG. 31 a) is formed inthe display section. An upper layer 53 of the gate line is formed on thelower layer 70. Thus, the lower layer 70 and the upper layer 53 of thegate line are electrically connected to each other (corresponding to thestep shown in FIG. 31A).

As shown in FIG. 33B, the gate insulating layer 54 is formed on the gateline and the gate terminal (corresponding to the step shown in FIG.31B). A part of the gate insulating layer 54 on the gate terminal is notremoved in this step but later.

Then, the TFT 57 is completed in the display section (FIG. 31C).

As shown in FIG. 33C, the interlayer insulating layer 60 is formed onthe gate insulating layer 54 (corresponding to the step shown in FIG.31D).

As shown in FIG. 33D, a conductive layer used for the reflectiveelectrode 61 is formed on the interlayer insulating layer 60(corresponding to the step shown in FIG. 32A).

As shown in FIG. 33E, the conductive layer is patterned to form thereflective electrode 61 (FIG. 32B) in the display section. Accordingly,a part of the conductive layer in the gate terminal section is removed.

As shown in FIG. 33F, a part of the gate insulating layer 54 and a partof the interlayer insulating layer 60 which are on the gate terminal areremoved. In the same step, the part of the gate insulating layer 54 andthe interlayer insulating layer 60 in the region T are removed in thedisplay section (FIG. 32C).

As described above, in the gate terminal section as well as in thedisplay section, the terminal and the gate line are covered with thegate insulating layer 54 and the interlayer insulating layer 60 untilthe reflective electrode 61 is completely formed. Thus, electrocorrosionbetween the gate terminal/gate line and the reflective electrode 61formed of different metal materials is prevented.

The source terminal section (FIG. 29C) can be formed in the same mannerin the same steps as those of the display section. Thus,electrocorrosion is prevented.

In the case where, in the display section, the gate electrode 52 and thegate line are formed before the transmissive electrode 58 a,electrocorrosion is effectively prevented. FIG. 34A shows across-sectional view of the gate terminal section formed in this manner,and FIG. 34B shows a cross-sectional view of the source terminal sectionformed in this manner. Both In the gate line and the source both have atwo-layer structure formed of a gate or source material and atransmissive material.

In this structure also, the gate and source lines are covered with atleast by a gate insulating layer until the reflective electrode iscompletely formed. Thus, electrocorrosion is effectively prevented.

EXAMPLE 12

After the step described with reference to FIG. 31C, the steps shown inFIGS. 35A through 35C can be alternatively used. Electrocorrosion iseffectively prevented in such a method.

As shown in FIG. 35A, the interlayer insulating layer 60 is formed. Thecontact hole 63 is formed in the interlayer insulating layer 60 byphotolithography. In the same step, a part of the interlayer insulatinglayer 60 in the region T is removed. The surface of the interlayerinsulating layer 60 is formed wave-like.

Next, as shown in FIG. 35B, a conductive layer is formed on the surfaceof the interlayer insulating layer 60.

As shown in FIG. 35C, the conductive layer is patterned so as to removea part thereof in the region T, thereby forming the reflective electrode61.

According to such a method, the transmissive electrode 58 a is coveredwith the gate insulating electrode 54 until the reflective electrode 61is completely formed. Thus, electrocorrosion between the reflectiveelectrode 61 and the transmissive electrode 58 a which are formed ofdifferent metal materials is effectively prevented. However, thetransmissive electrode 58 a is covered only by the gate insulating layer54 in this method. Accordingly, the method described with reference toFIGS. 31A through 31E and 32A through 32C is more effective inpreventing electrocorrosion.

Since the part of the interlayer insulating layer 60 in the region T isremoved in the same step of forming the contact hole 63, the number ofsteps is reduced compared to the method described above with referenceto FIGS. 31A through 31E and 32A through 32C.

EXAMPLE 13

An electrode structure for matching the electro-optical characteristicsof the reflective region and the transmissive region in a transmission-and reflection-type LCD device according to the present invention willbe described. There are two methods for matching the electro-opticalcharacteristics (voltage-brightness characteristics) of the reflectiveregion and the transmissive region. According to one method, thethickness of the liquid crystal layer in the reflective region ischanged from the thickness of the liquid crystal in the transmissiveregion. According to the other method, different levels of voltages areapplied across the liquid crystal layer in the reflective region and thetransmissive region.

The first method will be described with reference to FIG. 36. FIG. 36schematically shows a cross-sectional view of one pixel area of the LCDdevice according to the present invention. The LCD device includes acounter substrate including a color filter layer and a transmissiveelectrode (counter electrode), another substrate including a reflectiveregion 90R and a transmissive region 90T, and a liquid crystal layerinterposed between the two substrates. The transparent electrode isprovided in the vicinity of the liquid crystal layer, and the colorfilter layer is provided outside the transparent electrode with respectto the liquid crystal layer. The reflective region 90R and thetransmissive region 90T are provided in the vicinity of the liquidcrystal layer. Needless to say, the color filter can be eliminated.

The reflective region 90R includes a transmissive electrode 78 (e.g.,ITO), a reflective layer 79 (e.g., Al), and a transparent interlayerinsulating layer 80 (e.g., polymeric resin) provided on the reflectivelayer 79. The transmissive region 90T includes the transmissiveelectrode 78. The thickness dr of the liquid crystal layer in thereflective region 90R and the thickness dt of the liquid crystal layerin the transmissive region 90T are independently adjusted by changingthe thickness of the interlayer insulating layer 80 in each region.

Light used for display in the transmissive region is transmitted oncethrough the liquid crystal layer having a thickness of dt, whereas lightused for display in the reflective region is transmitted twice throughthe liquid crystal layer having a thickness of dr. In order to match theretardation by the liquid crystal layer in the reflective region withthe retardation by the liquid crystal layer in the transmissive region,the thicknesses dt and dr are preferably set so as to achieve therelationship dt=2·dr. For display in the reflective region, however,light incident on the reflective layer 79 at an angle as indicated bydashed arrows is also used. Therefore, the relationship dt>2·dr is morepreferable.

The second method will be described with reference to FIGS. 37A, 37B,38A and 38B. FIG. 37A is a cross-sectional view of one pixel area of theLCD device according to the present invention. FIG. 37B is a graphillustrating the electro-optical characteristics of the LCD device shownin FIG. 37A.

As shown in FIG. 37A, the LCD device includes a counter substrateincluding a transmissive electrode (counter electrode), anothersubstrate including a reflective region 90R and a transmissive region90T, and a liquid crystal layer interposed between the substrates. Thecounter electrode is provided in the vicinity of the liquid crystallayer, and the reflective region 90R and the transmissive region 90T areprovided in the vicinity of the liquid crystal layer.

The reflective region 90R includes a transmissive electrode 88 (e.g.,ITO), a reflective electrode 89 (e.g., Al), and a transparent interlayerinsulating layer 100 (e.g., polymeric resin) provided on thetransmissive layer 88. Since the thickness of the reflective electrode89 is smaller than the thickness of the liquid crystal layer, thethickness of the liquid crystal layer is substantially the same in thereflective region 90R and the transmissive region 90T. Accordingly, theretardation by the liquid crystal layer is different between thereflective region 90R and the transmissive region 90T. As a result, theelectro-optical characteristics in the reflective region 90R and thetransmissive region 90T are different as shown in FIG. 37B.

This phenomenon will be described with reference to FIGS. 38A and 38B.FIG. 38A schematically shows a cross-sectional view of one pixel area ofthe LCD device, which is different from the LCD device shown in FIG. 37Ain that the latter does not include an interlayer insulating layer. FIG.38B is a graph illustrating the electro-optical characteristics of theLCD device shown in FIG. 38A. In the LCD device shown in FIG. 38A, thethickness of the liquid crystal layer is substantially the same in thereflective region 90R and the transmissive region 90T. An identicallevel of voltage is applied across the liquid crystal layer by atransmissive electrode 88 a and a reflective electrode 89 a in thereflective region 90R and in the transmissive region 90T. Accordingly,the retardation by the liquid crystal layer is significantly differentbetween in the reflective region 90R and the transmissive region 96T.Therefore, the electro-optical characteristics are significantlydifferent in the reflection mode and the transmission mode.

In contrast, in the LCD device shown in FIG. 37A, the voltage is appliedacross the liquid crystal layer by the transmissive electrode 88 in thetransmissive region 90T through the interlayer insulating layer 100. Theinterlayer insulating layer 100 separates the capacitance. Even when thesame level of voltage is supplied from a driving circuit (not shown) tothe transmissive electrode 88 and the reflective electrode 89, thevoltage applied in the transmissive region 90T is smaller than thevoltage applied in the reflective region 90R. Therefore, as shown inFIG. 37B, the voltage-brightness curve in the transmission mode isshifted toward the higher voltage, i.e., closer to thevoltage-brightness curve in the reflection mode. As can be appreciatedfrom this, the voltage-brightness characteristics in the reflection modeand transmission mode can be matched to each other by adjusting thethickness, and/or the dielectric constant of the interlayer insulatinglayer 100.

The structure in which the thickness of the liquid crystal layer in thetransmissive region and the reflective region are adjusted can also beapplied to the electrode structure shown in FIG. 22.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

1. A liquid crystal display device for displaying an image and includinga plurality of pixels, the liquid crystal display device comprising: afirst substrate, a second substrate, and a liquid crystal layerinterposed between at least the first substrate and the secondsubstrate; a pixel region corresponding to one of the plurality ofpixels; wherein the pixel region includes a reflection region havinglight reflectivity and a transmission region having lighttransmissivity; wherein light from the reflection region and transmittedlight from the transmission region are utilized in displaying the image;wherein the reflection region of the pixel region includes a reflectionlayer provided at least in the reflection region; wherein thetransmission region of the pixel region includes a transparent pixelelectrode provided in at least the transmission region; wherein theliquid crystal layer is substantially thicker in the transmission regionthan in the reflection region; wherein the reflection layer is inelectrical communication with the transparent pixel electrode, and isalso in electrical communication with a transistor corresponding to thepixel region; wherein an upper surface of the reflection layer has aconcave/convex shape; and wherein the reflection layer is electricallyconnected to a drain of the transistor via an opaque metal layer.
 2. Theliquid crystal display device of claim 1, wherein an aperture is definedin the reflection layer, and wherein at least part of the transparentpixel electrode is located in at least part of the aperture as viewedfrom above.
 3. The liquid crystal display device of claim 1, wherein thereflection layer is located above the transparent pixel electrode. 4.The liquid crystal display device of claim 1, wherein the reflectionlayer overlaps at least one bus line which is in communication with thetransistor.
 5. The liquid crystal display device of claim 1, wherein theliquid crystal layer is at least twice as thick in the transmissionregion as in the reflection region.
 6. The liquid crystal display deviceof claim 1, wherein the reflection layer has a substantially rectangularouter periphery and the aperture is defined in a central portionthereof.
 7. The liquid crystal display device of claim 1, wherein thepixel region is rectangular shaped having two short sides and two longsides, and wherein the reflection layer extends along substantially anentire length of at least one of the short sides of the pixel region asviewed from above.
 8. The liquid crystal display device of claim 1,wherein the reflection layer has a rectangular outer periphery.
 9. Theliquid crystal display device of claim 1, further comprising aninsulating layer provided under the reflection layer, and wherein theopaque metal layer is located at least partially in a contact holedefined in the insulating layer as viewed from above.
 10. The liquidcrystal display device of claim 1, wherein the reflection layer overlapsat least part of said metal layer, and the reflection layer has asurface area substantially larger than that of the metal layer.
 11. Theliquid crystal display device of claim 1, wherein the reflection layeroverlaps at least part of the transistor.
 12. The liquid crystal displaydevice of claim 1, wherein the opaque metal layer is formed of a metalmaterial which is different than the reflection layer.